Oh, well for the monitor stand you will want thicker wood (takes fewer layers) so you’re looking at the thick option plywood or draftboard.
The thick ply and draftboard tends to run in the 0.200"- 0.216" range. I’d just design slots for 0.210" as an average value. 
Generally I design for snug fit rather than tight - nothing worse than having to sand everything for a too tight fit, but if you have the exact measurement, it’s still going to be a hair loose because of kerf.
I use these to adjust all tabs now and they do a great job of giving a nice snug fit without overkill.