A simple solution for 'small' engines. Weigh it dry, then weigh it wet.
Experiment - run a few minutes, then weigh....then repeat at regular
intervals, KEEPING A LOG BOOK (not for me, I keep it in my dusty, crowded
memory). Now you know how much water you use per unit of run time and can
easily calculate when the level is at a minimum safety level. What is a
minimum is up to you and your nerve.
Good Luck,
Wayne