happi I also use a gravel vac tube from the pet store. To make it more comfortable cut a section of a bike tube, about 2 inches is enough, and stretch it so that it fits over the end of your gravel tube. Put 1” over the end of the tube leaving 1” off the end. The free end will curl in. This rubber will then act as both a seal and also make it so the end of the tube is not sharp because it will be the rubber tube that is pressed against you, not the plastic. Just tape the rubber tube onto your gravel tube with some electrical tape and it will stay in place and last a long time. This should make it so that you could pump at pressures above what are recommended and still not have the gravel tube hurt you.
Just make sure that you lube the rubber when lubing your tube or dick so that it slides on easily.
I also use a small ball of wax to plug the other end of the suction tubing after I get to pressure. This allows for hands free while the vacuum is held for the 10 - 40 min or longer session.
If you do not have a gauge to determine what pressures you are using, I would recommend not only applying enough to expand your dick to a little less than your erect length. The less you pack your tube the further from erect length you will want to go. It is easy to over pump. I can go up to 10 hg before it starts to feel like a really strong pump and when I first started pumping with out a gauge I probably often was holding vacuum around 7 to 9 hg. It felt great but it started giving discoloration and caused too much fluid build-up.
I currently fully pack my gravel tube up to my head after about 5 min. of being in the tube, so, I believe that I end up getting more force pulling my unit longer instead of just girth. If you are not packing then you may need to not expand your unit as long because of the additional girth expansion that will be taking place.
I do believe that pumping without a gauge and pump can be safe, but, you have to somehow learn what is a good vacuum range, which is hard to do if you never have a gauge to check against.
On a different note, even if you add pumping to your PE, I would not remove the stretching from your routine. While I am doing some pumping now, my first inch of BPEL gain was through stretching and jelqing.