Hey Fred, I don't know about the "Princeton" fuel sender but here's my setup with the Vans capacitive plates. The inboard plate is mounted in the second cell of the fuel tank so it has no way of interfering with the flop tubes. The wiring wraps around the vent tube to prevent sagging and goes to its connector through the inner fuel tank rib. Proseal is applied liberally to all fittings and to the wiring on the vent tube.
The fuel sender is calibrated through your EFIS. I've got a Dynon FlightDEK 180 and the Dynon manual has the entire procedure outlined. Basically you add one gallon at a time and the EFIS records each gallon increment until the tank is full.
So far 240 hours with flop tubes in both tanks and no issues whatsoever. Fuel quantity indication is questionable just like a float sender. I don't think anyone has come up with a completely reliable fuel quantity indication system.
You can see the flop tube in the left side of the picture