For keycap clearance, I like to use a 20mm square for each single keycap. you can go a bit tighter, but it gets hard to remove the keycaps, as the wire puller doesn't fit between the cap and the case well. and you'll have less "wiggle" room if something doesn't line up perfectly. My method is to find the centerline of the top row of switches, and offset 10mm to get the inside edge of the case. You can go a bit more or a bit less to suit your preference.
For high profile height, I've been using 7.5mm from the top of the plate. less than 7mm will sometimes show a tiny gap at the bottom of the keycap.
I'm also leaving 3mm between the bottom of the plate and the case. You might be able to get away with 2mm, but some of the components on the bottom of the pcb could hit the case. If it's plastic, it's no big deal, but if you have a metal case, it can cause a short.