Hey Shawn, I have looked or tried to get some calibrations on and off, but kinda gave up. I think the WSB site used to have a chart years ago.
Anyway, I measure mine with an old weight scale and stretch the band down over it when they are attached to the pins in my rack. I get a reasonably close calculation that way. For instance my greens (Iron Woody's) are putting about #80 at top each=#160 maybe, the way I cinch them.
I agree with some guys that, I guess it does not matter a whole hell of a lot, if you calibrate light to heavy maybe. I get the feel with my empty bar and a band(s), then weight up from there.
Personally, I use chains a bit more, since the feel is a bit more stable or not as much of a recoil at top and very easy to calculate. But that is just me and I am no pro.