There a way to figure the converter slippage...
Some say slippage can add 300 to 500 rpm or more.
Logically the slippage must be more at full throttle than cruising speed.
A good street and strip converter will drive like a stock converter at light part throttle driving and then slip if you go to WOT below the stall speed, it should lock up and have less than 10% slippage at or above the stall speed, especially in high gear above 5000 RPM
Measure the circumference of both rear tires and inflate them so they are within 1/4 inch of each other on the circumference and then do the math for your RPM at the finish line in the 1/8 or 1/4 mile and see how much more or less your calculations show.
IE if you see 6600 RPM in high gear in the 1/4 mile and your calculations how 125 MPH and your time slips says 120.0 MPH your doing good, if it shows 125 MPH and your time slips says 105 MPH your not doing so good.
Anything more than 10% is not good, less is better.