I was going to ask the same question about the VacExtender. A part of my glans gets into the vacuum cap, so my NBPFSL gets shorter, because my glans gets shorter. Do I have to use the same principal with it?
Wuddup, I also use the VacExtender. This is my philosophy on it and what I've been doing. First, it's hard to measure while you are in the thing because your glans gets sucked up into the cap and in my opinion this kind of throws off the accuracy of any measuring while you are in it. I rely a lot on BPFSL to measure gains. Anyway, what I do during the time I'm in the extender, is I use a rod length that when I hook the cap to the top, my penis is stretched and the springs aren't fully depressed. This way I know there is tension on the penis because the springs are compressed. Now naturally after a while the penis will loosen up and I'll notice the springs aren't compressed anymore. At this time, I'll unhook and take a break for about 5 minutes or so, and during this time I will unscrew the rods some, usually about a quarter inch. Then it's just a repeat of the same process. I hook back in and let it work, check the springs once in a while, if the springs aren't compressed I unhook again, take a 5 minute break, unscrew the rods another quarter inch then hook back in.
I've never had to adjust more than twice over a 5-6 hour period. Basically you want the springs compressed and providing tension the whole time you are in the extender. Due to your penis naturally loosening up and stretching you will probably have to adjust a bit to keep the springs compressed.