Not quite sure what you mean. It's very unwise to pump without a gauge as you can get into get to dangerous pressures without realizing.
I'm not an expert (so feel free to ignore me) but I go in somewhere between soft and 50% erect and never exceed 5 inches of Hg of pressure. Most of my sessions are at 3 - 4 inches of Hg. My penis expands to slightly more than a natural full erection this way and is never painful or uncomfortable.
Be careful: it's easy to damage your penis when pumping.
When I pump, I go in erect, but come out in fat flaccid form. While I'm in there though it looks as though I'm staying fully erect. I think it might be possible to pump without a gauge if you use only the first pressure that keeps the size of your initial erection. It may be easy to go over this if you keep pumping past that point, but if you make sure you go in fully erect, I think it could be done safely with some practice. I can only really say this though because I've got a feel for the pressures precisely because I do have a gauge...