The stall speed of the converter is determined by the torque applied and the load presented. The same converter will have varying stall speeds depending on the motor. When you flash the converter from an idle, what does tach jump to?
Who made the converter? Does it have numbers on it? Call the manufacturer.
I bought a used Munsinger converter; called Mike Munsinger and told him what I bought. He asked the specs of the car, and told me it should be about 6000....I put it in the car and it went to 6200--pretty close to what he told me it would do.