work at 30, or 24.
Do not work at 12.
You should always work at the framerate you intend on working at for the rest of your flashing.
If you get used to 12 you'll realise it sucks and up to 20. then youll get to 20 and later realise it sucks and go for 24, then realise that that isn't the perfect framerate either and go with 30.
It's verry annoying to work with different frames per second all the time. So just animate with your inteded speed (Above 20).