I don’t’ think you should focus on the amount of weight you’re hanging. You know, like trying to get up to fifteen pounds. Like GM said, it isn’t a weight lifting contest and who knows at what weight you will experience fatigue. Just slowly increase the weight about a pound or so per week until you reach fatigue and are forced to lower the weight in order to complete a set.
For instance, at this moment I’m hanging 7.5lb on my second set for the day. I started this set at 11lb, hit fatigue around ten minutes into my set and had to reduce the weight twice in order to complete the twenty minutes. I just add and remove weight in order to ‘ride’ the fatigue, attempting to log as much hang time as possible in the fatigued state.
Fatigue may not happen for you until you’re in the 20lb range. Who knows?
Then (4.5 nbpel x 4.75 mseg)
Now (5.625 nbpel, x 5.25 mseg)