John,
Sorry, I work totally blind when using such things, maybe wrongly, but it seems to work.
I have a load of the tri sided self cutting screws, especially used, like you said, in thinnish materials, and I use a standard tapping hole for the size and that seems to work well for me.
I occasionally use thread forming taps, and I use the same standard sizes for those as well, but only use them in soft materials, never having had the b***s to try them in stainless, but supposedly they work well in that also.
John