When you're using a iron tool, the ROC of the tool doesn't change very much (it can and does move a little bit but that is a lot slower than the glass does) with one mirror so you really need to have the right ROC in the first place.
Once you go to a tile or glass tool, the ROC will change a lot faster and you do need to keep track of the changes to that ROC. Even at the 25 micron grit size, you can quickly change the ROC by sevral percent in a few wets.
FWIW, I have a pair of 2" end caps for iron pipe that the group's mirror grinders have been using and, after a fair number of different users (several years now) the fat ring around the end of the cap is about 2/3 gone. These caps have been used as ring tools for anything from 6" to 12.5" mirrors. This, I think, tends to indicate how slow the iron will move with grinding over the use on a single mirror - yep, it will move but not very far!