I'm in an area which I know very little about. I use alternate tunings a lot in my music. It usually isn't an issue to tune a guitar to an alternate with the strings that are on it. I have also been tuning bouzouki to different tunings. This is also very close to guitar scale length and doesn't pose a major problem for most tunings, being in the 24"-25" range.
 
Where I seem to be running into trouble is with my mandola. I don't really like the tuning on it now for what I'm doing. It has a weird scale length. The bridge is movable but if you go too far out you change the intonation on the higher notes, so it sits somewhere in the 18.5 " range. From what I gather if you tune with too much tension you'll snap the neck. Too loose and the strings are floppy. These are typically closer to a 17" scale length. Mine is a bit unusual compared to some others and is closer to an octave mandolin, scale length typically closer to 19-22". My plan is to tune it more like an octave mandolin and less like a mandola. . Mandolins are tuned the same as violins with short scale lengths. Typical tuning for it is CDGA  a 5th lower than a mandolin. I want a more D friendly tuning that compliments guitar DADGD and GDAD on zouk....
 
I've read around online and the whole thing is simple physics but it can get complicated. I tried to use the D'addario string calculator. It needs more info than I can give it. I mean, I need to know the string gauge not provide it. I also need to know the tension so I don't break my mandola neck. It seems that people who are doing this are using heavier strings to get those tones on smaller scales which can only be had within a small window and don't always sound correct. The formula takes into account your total scale length ( in my case 18.5" nut to bridge) and the tone you want to get from each string and the tension on each string. The end result can either be great or it can be a bad compromise. I finally gave up the other night and ordered a few sets EJ76 and J72 D'addarios. Though not the same as ordering individual strings, it seems to have worked for a few other players.I haven't found any definite tried and true ways to do it. Any suggestions or advice? Thanks!

This looks promising
 
http://stringtensionpro.com/

Starise,
 
I don’t have a direct answer to your question, but I do know that different manufactures strings can have different tensions for strings of the same gauge.
 
This became really apparent a few years ago when I decided to try flat wound stings on my Rick Bass.  When I first put a new set of Rotosound Flat’s on my Rick, the neck started to warp.  I quickly pulled them off for fear that I had damaged the neck.  When I went to Rickenbacker’s website, it actually has recommended string tensions for Rick Basses and will void your warranty if you exceed those recommendations.
 
That’s when I started to discover that different string types and manufactures, even though they are the same gauge, have different string tension.  Flat wound strings have more tension that round wounds.  LaBella’s have a different tension than Rotosounds, Fender, Thomastik, Chromes or any other brand string, in the same gauge.  Lighter strings, obviously have less tension than heavier strings. 
 
Not much help, I suppose…but maybe you can look up string tensions by manufacture.  

https://en.wikipedia.org/wiki/Vibrating_string#Frequency_of_the_wave
 
It's a pretty simple formula, but the problem is the formula uses linear density and what we are interested in is the gauge. I know D'Addario does provide this information for their strings somewhere, but most manufacturers do not.
 
But if the manufacturer of your favored strings provides tension information at standard tuning, you can calculate the linear density from the frequency, length of the string and tension.
 
And if you don't like the tone of a string at a reasonable tension, I would suggest a different string type or brand. 

BobF
This looks promising
 
http://stringtensionpro.com/

That is a cool site. I just walked through other for mandolin and it spits out tensions at the end after you choose length and tuning.

Hi,
 
On my bass, I got a set of Rotosound strings, and replaced the ones that were in there, which I could not define if someone plastered it around my neck!
 
Maybe it was that I do not have an amp to hear this, but I could not tell the difference. Bapu would probably say my ears are not tuned right, and Craig would say ... you did what? ... and Mesh might say ... you need new ears! But I have never been able to find anything different on strings, and it might be that I have never taken this to a stage or a louder volume ... I have no idea!
 
I guess I have some deeper listening to do ...

Thanks for the info guys! The first link is the one that I had trouble with since it doesn't give me the guages for my scale. It seems to want the gauges which of course I don't have because I'm changing the tuning. It is a great tool though for calculating basic guitar string needs based off of info you already have. A real biggie is the tension because too much and you break necks. My scale is so close to Octave mandolin that I think I can get it tuned in that range.
Drewfx1- That is a really helpful link. I believe I might learn something from it. Thanks to you and to all who helped on this. 
There are more variables than I had considered before. The make up of the string makes a difference too. Phospher Bronze, round wound, high tension steel or chrome...all have different characteristics.
 
I took the easiest and least educated route by simply buying strings and trying them. To do it right I'll need to get more up to speed on these calculations, tensions and such. Buying a set of factory strings almost always is a compromise to picking single strings based on a proven formula. A few mm one way or the other can make a huge difference using alternate tunings and getting the right harmonics etc.
 
If you get really lucky you'll find someone on the web who has a similar instrument to yours and is doing a similar thing with it. But in this case it's either study or trial and error withing certain limits.
 
Glad you didn't crack the neck of that bass Mike!

Starise - I actually design strings for a living. I have the testing equipment at work to get the tensions at tune and scale. I make mandolin string up to .040 wound, but nothing for a mandola.
 
If you want to PM me, I can give you my info and we can talk about what your looking for, in the scale and string gauge you might need.

Have you tried shooting them an email? Folks at companies like that typically jump on "cool puzzles to solve."

Here ya go! http://www.superstringtheory.com/ 
 
HTH.