I record kerf measurments whenever I start working with a new material.
Easiest way is to cut a 1" square, then measure it with calipers. The difference/2 is your kerf. Most of my 1/8" woods are .006".
Always use calipers if you want a snug fit…
When doing a perpendicular fit like these, take width of material minus kerf×2. Thats the slot width.
I generally design for whatever material I plan to use.