Hi

As to the formula in question it's rather a math-thing. If you can calculate the value for G2 on a piece of paper you can simple introduce it to excel.

What you got now:
M2 = (G2/1.2 - F2)/G2
From there you want to know the value of G2, let's have it done (in case you missed the class for whatever reason ):
M2 * G2 = G2/1.2 - F2
M2 * G2 / G2 = G2/(1.2 * G2) - F2/G2
---> simplified: M2 = 1/1.2 - F2/G2
M2 + F2/G2 = 1/1.2
F2/G2 = 1/1.2 - M2
1/G2 = (1/1.2 - M2)/F2
G2 = 1/((1/1.2 - M2)/F2)

It will give you a accurate result, whereas GoalSeek only provides a result by iterations. Calculation by iteration is stopped at a certain point, to achieve a result in reasonable time...
Therefore I spare what the macro must look like to achieve dynamically a less acurate result by GoalSeek