Hey Jim,
I dont want to say you are incorrect but I dont recall ever recommending to lower it below 0.1mL but I have used mine successfully with less. However, you need to have a very good seal to be able to trust using less and even though it might be more damaging to the piston over time using more black powder than absolutely necessary, I do think it is better to wear out the cutter than to wear out the rocket. I did have good success with pyrodex since it didnt seem to be as violent as black powder but 2 or 3F BP would produce similar results compared to 4F. But this was more important back when I was using aluminum pistons as their operational life was limited; the stainless pistons have shown much greater lifespans.
TLDR: Dont risk the cutter not shearing the cable using less than 0.1mL BP. If you do want to try less: ground test, ground test, ground test!!!