I’m interpretating Buby’ beliefs, because I suppose he’s pretty busy right now. He could correct me later. 1440 lbs/H is the optimal number, on average. This is based on the extender time x tension recommended use.
Given that extenders instructions says generally that a minimum of 4 h/day wearing the device is needed, anyone can do his calcs. The minimal number to see gains is 480, and so on.
Pound Per Minute Theory PPM
Reading that reminded me that there are plenty of ADS devices, which could help you reach your optimum number of lbs/H.
One more thing—you didn’t say whether you’re hanging SO, BTC, OS or otherwise. Perhaps you’ve just been hanging the same way for too long, and this caused you to plateau.