You're right by one thing: Yes, I did miss the reference in E2 to G2

But using my piece of paper a second time it says:
G2 = 1/((1/1.2-0.15-M2)/(B2+C2+D2))

Now the question remains if I got it wrong at some point. I don't think so, but it always makes me suspicious if somebody else says "it can't be done" assuming he/she tried on his/her own, and therefore knows what he/she is talking about. So please just check if the formula is doing allright, otherwise you probably will have to do with GoalSeek.