So you have film in the tank and cant take it out to test the fill up? So do a prewash, fill the tank with water to the top and pour it into a measuring cup. The right amount will be a lttle less than what comes out from this trial as you need some open space at the top to allow for agitation.
After you get over your excitement you can do a fill test with the top off and see how much it takes to cover the reel sufficiently, probably during your rinse cycle.