I use my biscuit joiner all the time to quickly make strong joints. With narrow boards like face frames, I used to spend a lot of time figuring out which biscuit size to use so the slots wouldn't be too wide and show. To avoid all that recalculation, I made a permanent reference block.
I cut slots for No. 0, No. 10 and No. 20 biscuits in a block of wood and recorded all the information I need: the slots' exact widths, depths and centerlines. When locating slots for mitering, I insert a biscuit and note its curvature. This way I avoid cutting slots too close to the miter's tip. I drilled a 1/2-in. hole through the block to hang it over my bench, but I often keep this handy tool in my apron pocket.