Need help to Calculate potential Needed.

So my manager like what I did with our Performance calculator so he wanted me to delve deeper.

One of the Stats that determine the Performance is our Surveys.

When we get a good survey it is valued at 100 points
When we get a Neutral survey it is valued at 0 points
When we get a bad survey it is valued at -100 points
your survey score is determined by the total value of your surveys divided by how many surveys you got. i.e. if you had 15 good, 2 neutral, 3 bad; you would have a score of 60.

I so far have made a spreadsheet that will let you know what your survey score is what I need to do is add in a section that will let you put in a Goal of what you want your survey score to go to and excel will determine how many good surveys you need to hit that goal

I.e. if I have a 60% score with 15 good, 2 neutral, and 3 bad; and I want to get to a score of 80% how many more good surveys do I need to get?
What would the formula for figuring this out be?

For my current formulas I have:
C10: Under my Promoters input (C9) is =C9*100
D10: Under my Neutrals input (D9) is =D9*0
E10: Under my Detractors input (E9) is =E9*-100
F9: My Survey Score Formula is =SUM(C10:E10)/SUM(C9:E9)

I need a formula for H9 that will calculate how many surveys are needed to make F9 equal to G9 which the individual employee will enter data into G9

I use Excel 2010 and have an intermediate expertise of Excel.