Best way is to build a test stand on which you can adjust the gap with a screw. Run up your turbine, and turn the screw until your thrust scale peaks, that way you will get optimum (static) performance from your turbine.

OR.............You can refer to the turbine manufacturer's instructions.............Wren specifies 10 mm for the MW-54 MK I and MK II. For the super Sport, it's 25 mm.................

Tailwinds,

John