you can do a formula on slippage BUT the case size the stator and how they build the convertor with may years of building them tells them what would work in your vehicle .it not a one size fits all. i use coan you have to know were your engine is going to make power first build the engine .then get the stall made for the power and rpm cid cfm cam size weight trans gearing rear gearing tire sizes of your vehicle .i do not worry about slippage i call them have them build me a stall for the job at had