It’s fine too do what you are doing. It’s unlikely to be harmful. From your description, you’re essentially stretching with a little extra attention to the blood flow. If it’s working for you I’d say keep doing it.
It’s true you’ll hit more of your girth with higher levels of erection, but perhaps that isn’t the point of what you’re doing.
At any rate, to answer the question: no. I doubt you’ll injure yourself this way, and it may well help you gain length, which seems to be your goal.
I say have it.
Before: I'd like to show you something I'm very proud of, but you'll have to move real close.
After: I\'d like to show you something I\'m very proud of, but you guys in the front row will have to stand back.
God gave men both a penis and a brain, but unfortunately not enough blood supply to run both at the same time. - Robin Williams (: