Thanks.
To be honest, I haven't spent a lot of time trying to fine tune the focus, it's more the case of getting the LED high enough into the opening so as it doesn't create artifacts from light hitting the side of the hole. I think some guys have sanded the reflector base thinner to get around this also.
There's not a lot of room to move (adjustment) between getting the LED high enough, & sitting the reflector flush on the board. Short of making a focus testing rig, there's going to be a bit of trial & error measuring any slight differences in height (taking throw measurements).
I'd say get the LED as high as you can to begin with, then gradually add thin insulating gaskets to the reflector base/board, & measure each time.