Put a pice of perspex. metal etc in the lathe and face it off, anything doesn't matter as long as it's flat.
Then replace the tool with a piece of square bar [ lathe tool turned over ? ] and offer this up to the faced off disk in the chuck when it touching along it's length tighten the toolpost etc.

Now clamp you stepper with perspex faceplate to the lathe bed with a G clamp on some packing blocks so it's on centre offer it up to the preset lathe tool or bar. Tighen stepper when touching all over.
the stepper is now square to the lathe axis, change the tool round again, run stepper and face off.