I'm going by memory here but I remember Byron advising me to get the clearance as little as possible. I remember taking the impellor and sanding it face down on a sandpaper laying on a flat surface to get it even (it is slightly off in most cases) and then taping a piece of the same sandpaper to the impellor face while mounting the engine. Once in position, mark for bolt location.

The less the clearance, more the performance.

Hope this helps.

Adil