On the tank monitor - holes in tanks are bad... Sensors get damaged, corroded, and it you only have a few sensors the accuracy is bad... F, Half, E...
Take a look at this, it's the strips on outside of tank - it's everything your building, but already done! Lol
It monitors to the exact percent using a calibration of full capacitance and the empty capacitance. You can even set more markers for it to calculate better for irregular size tanks.
They make one for diesel tanks too!
Just saw a bit of discussion on the tank monitors
it's fun to build stuff too, so your Adrino idea is slick to get consumption rates...