I use a ruler, then do a test cut on piece of thin ply, measure than and adjust if needed. It's ok looking at a ruler(or tape) but how it actually cuts will likely be different if theres a tiny bit of wobble or runout.I've never trusted my table saw rulers and always check with a tape. Though the ruler is handy if you just want to adjust by a known amount, e.g. slide the fence exactly 9mm etc