I used this to calculate mine

More HP/torque will increase stall speed
Heavier the vehicle, the higher it will push the stall
Lower gears, such as a 2.73 or 3.08 gear, will increases stall

The mathematical constant that defines a converter's stall speed (in rpm) is called the "K" factor
K is the observed stall speed of the converter divided by the square root of the applied torque
The relationship is expressed mathematically: K = RPM/torque^0.5
Once the K factor for a converter is determined, the stall can be predicted for different torque values