I believe you need the pressure from a pump to get an accurate reading. The ones I have seen have the same type of setup internally as the ones that tell you how fast your boat is going which when compared to gps speed are very inacurate. So like already said putting them inline with the gathering tank to the holding tank will work. But so will some measurements with a ruler and then sticking a yard stick in your tank and knowing how many gallons there is in an inch. We have a chart written down and a quick dip with the yardstick I know how many gallons I have or how many I have boiled. Yardstick was free at county fair.
2X6 deluxe Phanuef
Adding 200 more every year
27 years left of building a Hobby into a retirement time burner.