Don't completely rely on those numbers scotty. I use carboy handles, and with them the cap doesn't fit all the way on, so I have to hold them on with my hands. With my MityVac, I have a gauge and will "allow" it to cave in around those numbers. You can play with yours and come up with the same results. However I would guess, safely, that if you fit the cap on snuggly, then you could watch the top of the orange cap sink in, at that point, as memory serves me (poorly these days) you'd be in the 18HG range. I'll have to experiment for ya and take some pictures and post for you.