The easiest and often quickest way to set the height adjustment of router tables and table saws is to use setup bars. Setup bars are metal bars that have been precision cut to specific dimensions. Not unlike metalworking gauge blocks, woodworking set up bars are often longer, set to fractional measurements, and machined from brass or aluminum stock.
In my experience, setup bars are faster and more accurate than using a measuring tape, quicker than making trial-and-error cuts, and easier to use than setting cut or routing depth with a large wood workpiece or sheet of plywood.
Setup bars are much handier and more commonly used for setting router table bit height, but I’ve found them to be useful for setting table saw and circular saw cutting depths as well.