thefreeman did a thorough study of the slicing effects in beam profile and intensity for the LH351D emitter:
LH351D slicing measurements : output, tint, tint shift, beam profile, intensity.
Esentially speaking 1mm is the sweet spot, with the highest intensity gains and the most consistent CCT and DUV vs angle results.
Just get some 1mm washer(s) or a feeler gauge set to aid you in guiding the razor blade.