looks to me like she is brewing with extract. using the recipe would be much closer.
if you just use the FG, you are dealing with attenuation and which yeast is used, how far it went, what type of extract was used, what (if any) sugars came from the grain, etc.
seems using the recipe would be MUCH more accurate from extract. i would never try to calculate OG for a PM or AG recipe...there are just too many variables including mash temp, yeast range, etc. i'd just wait until i was sure it was finished before bottling or do it to taste for kegging.
what if you wind up with a fg of 1.020 and it doesn't move for a few days but it tastes ok? should i just assume my og was 1.080? there would be no way to tell when your beer is done if you don't know the OG and that, in turn, means that you cannot calculate the OG from the FG.
the metric madness calculator seems to deal with attenuation and estimated OG based on your system and what grains you have, not calculating OG based on FG. it doesn't mention FG at all.
sorry, just sounds like an unreliable way to do it to me (and again, an extract recipe would do the trick)