Forget it.... I know this sounds harsh, but it's the only honest answer. CoDecs that use variable bitrate allocation, variable temporal spans and multiple levels of spatial compression are literally mathematically unpredictable - you don't know the result until you have run the clip through the algorithm. The effective data rate too much depends on the picture content. That's why in encoders it is always called "target bitrate", not final output bitrate. Only CoDecs that use fixed matrices, regular intervals and clearly structured visual compression algorithms can be calculated such as MPEG-2 and H.264, but even there is always a minor variability and uncertainty. It may not be convenient, but testing out settings for each clip is pretty much the only way, if you need to achieve a specific file size. As a rule of thumb you could base your guesstimates one third below the desired size/ data rate, i.e. if you need 5MB files, target your settings at 3.5MB.
That in itself is good to know. At least I know now that I'm not missing an important shortcut.
I'll go with my vague equation, and when it doesn't work I'll just do it again.
One thing to keep in mind is that you can decide what's important to preserve in the movie and then throw away a lot of the rest.
I assume that the only reason that you'd be emailing a movie is for client review. So, what are you asking the client to review? Colors? If so, then frame rate doesn't matter, and you can drop the frame rate down to something really low. Timing and smoothness of animation? Then keep the frame rate high but sacrifice image quality and compress the bejeezus out of the images.