You don't seem to be getting an answer to this, so let me take a shot at it. I am not a lapidarist, so you know what my answer is worth.
I don't think you are going to see the "flash" until the rock is cut and nearly to the polishing stage. Although if it is wet it may help. The effect you are looking for comes when the cut and polish is parallel to the principal cleavage plane in the labradorite. As near as I can tell, the orientation of the crystals in the rock are random so it is "guess and try". If you can see more cleavage planes on one side of the rock than on others, you might try cutting parallel to that side.
There is a labradite rich rock from Scandinavia (Norway?) that is commonly cut into kitchen counter tops and facing stone for buildings. Despite the large quantities imported, I understand that something like 90% ends up as road fill because the cut faces do not have enough schiller.
From Don Peck - January 26, 2008 at 22:01:49