"really easy"... I haven't even heard about Quaternions before and I have studied electrical engineering for a while... =)
Ahh, but that's the joy of it. With the tools Peabody is talking about you get the benefits of math like his...without having to actually know any of it
Let him deal with the Quaternions (Admittedly not that common in EE since you're not usually dealing with as many dimensions!) and such - then when his tool is done just plug in your motor positions and angles...and voila - you've got mixer settings.
Kind of like the spreadsheets that Joshuabardwell did for figuring out the mixers in MW/Naze on his spider - only with heavier math to help deal with things like tilted motors as well as motor position.