For pressure, when I first got my Might Press, I had to rotate the knob at least a half a dozen times to get it to where it was even applying *any* pressure to the upper platen. These things must not *arrive* set up in anyway.
I repeatedly turned the knob a full rotation, closed the press, checked pressure. I continued one rotation at a time, closing the press each time, to see how hard it was to close. At one point, I did reach a point where it was very difficult for me to close the press - even with 2 hands. To me, I found the upper end of the highest pressure.
I know my press warranty states any damage caused by too much pressure will not be covered in my warranty, so I figured going to that upper range of high pressure was what I decided I want to avoid. From there, I found a place that I felt was med-high pressure, safely below the area that could damage parts, but apply enough pressure to get the job done.
There is no science to it. Just a gut feeling, and then how the shirts looks and wash after being pressed.
I have absolutely no problems with my finished products, so I figure things must be going okay.
Rhonda is absolutely right, imo, that when you open the weave by stretching, you will see the white lines, but when you release the stretch, it should return to normal. That's what makes JPSS better than other papers. Other papers, once cracked, do not heal themselves, they stay cracked looking.
In addition to stretching the shirt right after a hot peel to open the weave and re-pressing with teflon to re-set the ink... in addition to that - I also give the shirt a gentle pre-stretch *before* I do the pre-press to remove moisture. A few other folks do this, and we seem to feel it opens the weave even *before* the first press applies the ink. Some folks also think it's helps open the weave to remove the moisture better.
So I stretch the shirt, pre-press the shirt to remove moisture, peel hot, stretch the shirt, repress the shirt, and I have no issues with cracking. Even when I stretch the shirt a bit, the white weave is not that bad, and it doesn't show until you really stretch it.
To me, the white weave showing is even less then when you only stretch the shirt *after* the first press. It could be the brand shirts I use. Shirts make a big difference, but this is what I've seen. Once in a while when I comment on doing a pre-stretch, a few folks who also do this agree it helps even more... so, it's an experiment for anyone else out there who wants to try their luck with that extra pre-stretch as well.