Associating a weight figure with a pressure reading

Hello everyone. I am striking out on associating a PSI to weight readout I have. What I have is air springs on a piece of equipment, and a pressure transducer reading the air pressure in the circuit. I have the pressure sensor reading accurately and smoothed the way I want it. I have attached a picture of a graph I want to add into my sketch as another readout. I have an accurate pressure reading which I am calling “pressure”. I tried to get close with this equation … Weight = Pressure * 3.54. The Minimum on the graph is 15 PSI = 50 Lbs, max is 120 PSI = 450 Lbs. So I divided 50 by 15 and 450 by 120, then averaged the results and came up with 3.54. So basically I am averaging the conversion factor. It’s close, but I’d like it to be more accurate. What is the best way to go about this?

Multiply by 3.81, then subtract 7.15 from the result.

Fit a straight line to the two points (15,50) and (120,450).

Slope is (450-50)/(120-15) = 3.809

Offset = 50 when PSI = 15

pounds = 3.809*(PSI-15) + 50