Percentage Target Formula

Can anyone help with helping me to find a formula to speed a frequent task up. It is a percentage based formula. What I have is a column that has average sales (C) and a column that has a "current/proposed draw" (F) and a column that has the "NEW suggested Draw" (G). I use a column to calculate the current percent Efficiency (E) (=c/f formatted to %) and my goal is always to target a certain percent efficiency which I use a column (H) (=c/g) to help me aim for it. With thousand of lines to go thru I am wondering if there is a way to create the formula in column (G) that will end up at my targeted % eff that I would normally show as the percentage in column H. I am not sure there is a way to do this.

Normally in Column G I would enter a suggested draw (say trying to reach 40% eff in column H) and keep tweaking that number until it was a close to 40% as possible. It would be so much easier and quicker by light years if I could just say in col G somthing like c/?=40%, I am not sure if another factory column would need to be added, I just can't wrap my head around trying to figure this out lol.. thanks for any help.

The number in G would be a number and not a percentage. I would still have column H to show that target in percentage form.