Need help with gearing selection on bracket car (28" tires, C4 auto, N/A 347)

[93trickflow5.0]

I have a 94GT with N/A 347, C4 automatic, and 28 x 10.5'S tires on the rear. I want to shift and go through the traps about 6400-6500 rpm. The Coan 9" convertor flash stalls about 4000 rpm. I found a calculator on the web to figure this, but it wants me to enter a value for convertor slip, and I have no idea what that is. I can't believe it would be more than 2-3%. Guys that I have talked to tell me that it is a crapshoot the first time. You have to try a ratio that is close, and adjust from there. Well I'd like to buy just one set of gears if possible. The car has a 9" rear. It currently has 3.50's which are way too tall (it was a street car before). I also have a set of 4.88's which I think will be too steep. Anyone have a good way to figure this out?

[Thriller8]

What do you want to go through the traps at? With the cam I am running I should go through the traps around 7700 so when I bought a custom cam from Jay Allen he told me to go with 4.88's. I think it all depends on what rpm you want to go throught traps with.