: use a
: permanent marker and mark the water level on the outside of the outer cup.

drill a marker pen sized hole thru the marked line(s). put the unmarked cup back in the marked cup, put the marker tip thru the hole and spin the inner cup all around 360deg.

repeat as necessary for other lines or other cups:

now the complete circumferencal line is way easier to check that the resin level even from other eye angles slightly better than a simple mark on an outside cup. like you can tell at a glance if the cup is not straight up as the line or gap is not even all around.

also all cups used can be easily marked for double take assurance.

as well as the template cup doesn't get quite as abused and maybe messy as it is not used in any of the resin transfer process.