I've actually done an Excel spread for this. You have to figure out how much of a time duration each beat takes up in the current (old) meter. Then once you know that, you need to know how many beats in the old meter should elapse while the new meter plays. Now you know both the number of beats that you're trying to cover and the number of beats that you're trying to sound in the new meter. Assuming that you're not slowing down or speeding up the feel, the new tempo has to correspond to the number of beats that you want to pulse over the target duration. I'm not sure all of this made sense, but that's what I've done to figure out tempos where I go back and forth between 3/4 and 4/4 or between 12/8 and 4/4.

Good luck,
Bill