Measure tap-to-tap on the boiler with a tape. "Required water level" is where the operating level should normally sit, measured down from the top (reference) tap.
Transmitter
Use 0 unless directed to use a different reference value.
Controller scaling
Custom setpoint (zero-based)
Match transmitter exactly
0 means the controller reads zero at the operating level and shows +/− inches from there. This is the standard method.
Controller will display the exact same inH2O scale as the transmitter (matches SCC Inc.'s "Method 1" from their level control guide). The setpoint will show the raw distance value instead of zero.