Gaining length while packing is possible , but it takes patience and attention. My current routine is directed toward this effort.
I used to suffer numerous blisters because I was not disciplined and refused to stop when I had exceeded my limit.
I got fed up with that and now stop short of that limit and have not had a blister for a couple years.
You can usually feel the onset of a blister. The glans begins to sting . It also turns white from lack of blood circulation and becomes firm to the touch.
When this occurs it is past time to stop. It is possible to prolong your pumping session, but it requires constant breaks and massage/jelq to restore circulation.
You can pull for length at higher vacuum but you cannot remain at that level. I progressively pump up to maximum stretch and immediately back off the vacuum. By repeatedly cycling the vacuum up, and down you can increase your length up the tube.
When I started I was excited for reaching a milestone of 7” in the tube. Years later I was regularly pumping up to 7 1/2” and holding it there.
Now I am comfortably pushing 8” , but park it at 7 3/4” to stretch, and back off to 7 1/2 - 7” to relax. So I can see some progress, but I am trying harder to make more consistent progress without stressing to the point of injury.
Just take more frequent breaks, don’t hold at high pressure, and always keep your glans soft and pink.