After the initial firing in your rifle, headspace off the shoulder and not the belt. I don't own a 7RM, but I do shoot four other belted rifles. On these rifles, I pump the shoulder back 0.001-0.002" when sizing, that's it, and no more. If you don't have headspacing set in your reloading gear, it is time you purchase one. I pretty valuable piece of equipment in my opinion.
Also, one other piece of equipment regarding belted mags that has come in handy twice now is this gem of a tool.
http://www.larrywillis.com/ Belted Magnum Collet Sizing Die If you use your brass enough, you might need it in the future. I had a 300 Win Mag that is now down the road, but it needed the brass ran through it after about 5-6 firings. I have used it once on some 375 H&H brass after the 3rd reloading.
Good luck!!!