Dunno if you can weigh the res, but H20 is ~8.3 lbs/gal. Or:

I stuck a BBQ skewer thru a wine cork and made a "bobber." Drilled a small hole in the top of the res cover for the skewer to stick out and put marks on the skewer at .5 gal intervals. (A Wee Zard invention.) :thumbsup: