"Roll your own Audio metronome"
http://www.sightsea.com/music/metronome.html

I've been doing this for more than a decade and it's the only way to roll ... for me.

I'll give it a try to use 3/8 for recording. I'm not sure it will work with all my sampled instruments, especially with pre-recorded phrases synced to host bar and tempo.
 

4/4 triplets isn't always exactly correct.  Contemporary 12/8 tempo is usually specified as beats per dotted quarter.
 
How does Sonar compute the specified tempo with 12/8?

What about notation later?
 
Can you fix that from whatever is recorded - divide it down?
4/4 with 8th note triplets or something?
 
Setting grid to 12/8 - is that going to be correct then?
 
Haven't looked into latest improvements for v21, if any.
Nice if you can catch that as well from what you recorded.
Thanks.

A little more on the roll your own metronome idea I use:
I can manipulate the mix (volume and pan) of the clicks with envelopes, switch from 4/4 to 3/4 and have it follow my changes with appropriate accents on 1. That's done the same way with volume envelopes and multiple tracks.
At it's simplest it would be 4 tracks, 2 for the 4/4 and 2 for the 3/4.
More tracks can get into the "1 e and a 2 e" with accents on any part of the beat and also implying a 3 over 4 feel.

...Just Thought...
 
If you are just looking for a simple  click track so you can set your grid to 12/8 without
worrying about midi lining up later;
 
1.Write a 12/8 hi hat loop with the accents you require in AD2 or Session Drummer
or whatever you prefer(AD2 and Session Drummer allow writing INSIDE the program itself) .
 
2. Drag the loop to an empty place in the track view and enable Clip Looping after
Sonar creates a track for it, and drag it out to taste.
 
3.Bob's your  Uncle.
 
(also, I am relatively sure you can find a stock 12/8 loop within either of these and change the accent points  at will.)
 
Hopefully this might save you some math.
 
Mel

jerrydf
I'd go for 4/4; 12/8 is basically four triplets every bar. But RobertB is right - create your own click/percussion track.  I never use the metronome.

 
I agree. 4/4 with 8th triplets. That's how I do 12/8.
 
If I need a metronome I make up an appropriate rhythm consisting of high-hat, snare and bass drum. Sometimes I add some synced delay. I hate the built-in metronome and don't ever use it. I never has the right "groove".