@VigII posted:The next step after the bubble level is to enter the depth of the tank. I didn't worry about that because the reading will only accurately reflect the gallonage if the volume is known. I used the hose end meter I always used. I filled the empty tank with 11.25 gallons of water. 11.25 gallons equates to 25% of the 45 gallon tank. I varied the tank depth inches until the Mopeka Pro read 25%. I proceeded to 15 gallons representing the old light set of 1/3 of a tank and the Mopeka Pro read 33%. I was very happy to see this level of accuracy. It's been that way so far each time I've filled the tank to various levels.
I like this idea. Nicely done. This is probably now on my list. 
One more thing you may want to try out in the calibrating step... I noticed, when I was calibrating my "sight-glass" mod, is that my tank had several unusable gallons at the bottom. Three I think. I don't know if that is a normal condition or not. I figured that out, when after draining the tank completely and de-priming the water pump, the pump would not re-prime until there were 3 gallons in the tank. I did not test the other direction ... in other words, if the pump was still primed, would it pick up the last three gallons? In your set up, it might be an easy thing to test ... when Mopika says there are only 5 gallons left, turn on the pump and see if how many it says are left when the water stops flowing. Just a though.
Side note: The "sight-glass" still works and is way better than the indicator light for that bottom 1/3rd. But, it isn't very good when you get down to the last 5ish gallons... and it is kind of a PITA to keep clean. I think I may give Mopika a try.