Pneumatic Differential Pressure transmitters cannot measure in the negative so you need to change the HP and LP sides around so that the HP side goes to the top of the vessel and the LP side goes to the bottom tap off point on the vessel. You now need to do your calibration in the reverse.
Steps to Calibrate

Find the ATM value first, in other words max positive differential (HP wet leg filled and LP open to atmosphere) on the transmitter this will now be your ATM value will say (+1350mmH2o).

Actual zero will now be 20mA and not 4mA and will be determined by making use of the ATM value minus the actual zero measured value, multiplied by the liquid density.

The actual 100% value will be determined by making use of the ATM value minus the actual 100% measured value, multiplied by the density. You should end up with something like this, zero = +1250mmH3o = 20 mA and 100% = +150mmH2o = 4 mA.

Finally the display on your remote level indicator, this needs to be changed as well otherwise it will read in the reverse.

If you use a pneumatic Differential Pressure Transmitter just substitute the 4 and 20 mA with the 20 to 100Kps or 3  15PSI the principle stays the same.