Rack and pinio preload

hello everyone

I am in process of replacing my machine drive from belt to rack and pinion similar to f-117 design and i am wondering how to calculate the preload on that, I am using mod1 10mm x 10mm 20 degree rack and 20 teeth pinion with spring tension my motors are 170 oz from OpenBuilds part store.
Also how to convert it to the turqe I need to apply on the screw with moment wrench?

Thanks for all the help
Amos