You can use built in Melodyne Essentials to calculate tempo exactly.
 
Also read this thread too. Niles has done a good explanation here as to how to calculate tempo.
 
https://forums.presonus.com/viewtopic.php?f=151&t=28030
 
I use a Boss metronome which has got a great tap facility. The tempos are usually whole numbers of BPM and are pretty easy to figure out.
 
Thanks to Dan too reminding about the crosshair thing. Mine has been off for so long I have forgotten about it. But it is there and can be used when greater accuracy is required.
 
 

Thanks Jeff, that's cool.  I mostly do instrumental stuff and probably would not think of using it for that.
 
Also, I'll second your recommendation on the Groove3 video regarding MIDI editing in SO.

Once you do figure out the tempo and put into the tempo setting in the clip it will stay there. It does not take much to quickly figure these things out and insert the tempo info. Then you will be able to always have those clips adjust to session tempo rather quickly etc.. I have got large Acid loop libraries myself from a time I was teaching hip hop production to young juveniles in detention.

Magic Russ
Currently the biggest issue I have is that apparently a lot of my "Acid" files don't have tempo info in them.  As a result, some of these loops don't line up with my measures.  If I can guess the tempo from the filename or folder, I can set that in the imported loop.  However, if I don't know, I can't get the loops to line up correctly.
 
I haven't had this problem with REX files.  I suspect that unlike .wav files, the .rx2 format requires the tempo information to be stored.

If the Acid file doesn't follow tempo changes, then it's not really an Acid file and does not contain the needed metadata. Sonar lets you create Groove Clips within a project, and they behave like Acidized files. However, if you have a standard WAV file, bring it into Sonar, and convert it into a Groove Clip, the original WAV file remains unchanged. You need to save the Groove Clip from Sonar, and then the saved version will include the metadata. 
 
Studio One handles the tempo aspect of Acidized clips perfectly if you choose Time stretch mode, but it does this on its own by reading the file tempo, not with the Acid metadata. So it cannot include pitch markers for transposition - a fairly glaring omission, because that's a major advantage of Acidized files over the REX format. However, as a workaround for both file types S1 makes transposition relatively easy. The only issue is after you transpose a clip, you can't roll out it out to create repeats so you need to use the duplicate command on the transposed clip.

Jeff Evans
What a lot of people are forgetting is that Studio One is only 6 years old and other DAW's are like 30 years old. None of them were even remotely as powerful at 6 years old as Studio One is now. 

 
Jeff, I truly appreciate your enthusiasm for a fine program and of course, S1 has made great strides in 6 years, but I think the efforts of the early DAW pioneers shouldn't be so easily dismissed. Remember that when Cakewalk's DAW was 6 years old in 1993 VST virtual instruments didn't exist, there was no ReWire, no acidized or REX files, no VST effects, no auto-tune, ASIO wasn't an open standard, the 66 MHz Pentium had just been released and cost almost $878, the cost per gigabyte for a hard drive was around $1,000, and the DVD-ROM didn't exist yet (delivering Studio One's content alone would have required something like 40 CD-ROMs).
 
This obviously takes nothing away from Studio One's rapid development, but there's no way a DAW in 1993 with six years of development behind it could be anywhere near as powerful as a DAW in 2017 with six years of development. The DAW pioneers (some of whom were/are involved with Studio One) paved the way for us to enjoy the level of performance and features we have today. Like today's software engineers, they did the best they could with the resources available at the time. 
 
 

Anderton

Magic Russ
Currently the biggest issue I have is that apparently a lot of my "Acid" files don't have tempo info in them.  As a result, some of these loops don't line up with my measures.  

If the Acid file doesn't follow tempo changes, then it's not really an Acid file and does not contain the needed metadata. [snip]
 

 
The quotes around the word "Acid" was supposed to imply that they weren't properly acidized, even though I might have reasonably expected them to be.

I am sad as well just like many Sonar users as to what has happened. It was Sonar Producer 8.5 that actually got me out of a full blown synth hardware setup and got me into the direction of replacing it all and moving towards the incredible virtual setup that I have today. (I still have some hardware though!) It did an amazing job of that for me.  
 
Yes of course Craig I get where you are coming from.  I guess the point I was trying to make is that even in the last 6 years or so since Studio One has come out they have made some huge developments from version 1 say to where it is today at V3.5.  They are own their own path and I don't get the vide they are trying to copy what everyone else is doing either.  Once you see what they doing and how they think then it all starts to fall into place somewhat.
 
We are all pretty excited too at the prospect of you being involved with Studio One as well. I am sure you are going to learn it pretty damn fast and as per usual you will be coming up with very interesting ways to use it.
 
There are also some very knowledgable people on the Presonus forum as well.  Studio One Expert is also a great site too.
 
Happy New Year to all and I hope this 2018 year will be prosperous for all no mater what DAW everyone decides to get into. Helping out here in this thread has been pretty good for me too. I have found out quite a few things I did not even know myself!