Red hit the nail on the head here, and it's been a real problem with stretching (as opposed to hanging). How much force are you using? How much force
should you use? Mostly people have had no idea. They can't tell us how hard they're stretching, and we can't tell them if they're stretching too hard or not hard enough.
In case you haven't seen it, you should check out my thread on this exact subject (the link is in my signature). I strongly encourage you to start doing this so you'll know exactly how hard you're stretching, and the whole community will have your data going forward.
Going by feel is not bad, and it's obviously what stretchers have been doing up to this point, but it can be deceptive. I started stretching at 20-25lbs of force and it felt GREAT... and also resulted in some injuries that forced me to take time off. Now I've backed off to about 15lbs force on my stretches, and I'm still getting a decent stretch but without the injuries.
The scale I use was $7 on Amazon. Pretty good investment.
Okay DLD but those frames are AWESOME, and you are rocking them.