Midgeting

I don't know of any guides. Since no one else has posted, I'll say this much. As far as I know, you can take as much off the back as you like so long as you leave the hole for the trigger frame. Just cut it off flat as any angle or design you try to do with the back may result in difficulty installing the IVG. Some people cut the back of the body to be flush with their trigger frame. Some people just take a little off and still leave the hole for the beaver tail.

You do need to consider a couple of things though. If you don't have a halfcocker, you'll need to shorten your pump arm, probably shorten your cocking rod, and shorten your bolt (or maybe have a custom bolt made). If you take too much off the back, there may not be enough length left to install your IVG and not have your hammer and spring bottom out in the back before the hammer gets far enough back to catch the sear. You may have to choose a different hammer spring.

If you have the mQ valve, it's easier. You can cut the back flush with the trigger frame. You'll either have to cut the spacer that goes behind the solenoid shorter or you can use a small stack of the right sized washers (I used 1/4 inch washers, but every hardware store's washers are a little different in size, you may have to choose a slightly different size). If you use the washers, you won't have anything to thread a cocking rod into, if you feel you need the cocking rod to guide the back block (not a problem with halfcockers).

If you have a halfcocker with mQ, then you can just cut the back of the body and install a few washers. Nothing else is needed.

Note that after cutting you will have bare, unanodized aluminum on the back. You may want to reanodize the body if you are unhappy with the appearance.