Boring, but really useful. Cut them out of the material you want to check. Slot you material into the slots to find the right size and you already have your kerf allowance!
The dimensions are correct in the drawing program, but the actual cut sizes will vary according to the kerf of the cut. So if you are looking for a tight fit, just try the slots until you find one that is tight. That is the dimensions you need to use in your drawing program.!