These scanners aren't scientific grade, so the lens height from the factory actually vary some.
However you'll have to test that yourself, the glass fluid tray for one person might be perfect, for another it might be off.
There is no way to adjust the lens height, only the film plane height.
Remember there are two lenses that focus at two different heights so be sure to select the ideal one for your scanning process. But yea, you sometimes have to shim the holders to get proper height, and Betterscanning holders are much better than the Epsom ones for adjustment.
Good luck,
~Stone