Those holes are the only ones I've done and they were 8.5 mm deep.

I don't know what "thread %" means. :/ I did a quick search and it looks like it refers to the size of the pre-drilled hole. For these holes I just used the closest drill I had to the recommended 4.2 mm drill size (a #19 bit, which is less than a thousandth larger). That reminds me: I was intending to put that info in the YouTube video description.

One of the nice things (or maybe handicaps?) of Procunier tapping heads like this one is that I do not believe there is a clutch adjustment. This one is suppose to be able to go from #0 through 1/4".