Need to calculate 5 star reviews for given rating in excel

Dear All,

I would like to have an excel application built that will determine exactly how many 5 Star online reviews are needed to achieve a near to 5 Star Reputation Online.
In fact, we cannot limit online review to 5 stars only, so we have all 1-5 Star rating, but to find out how many 5 star reviews needed to achieve our goal most near to 5 star reputation for that I need a formula, I am aware that in some cases a 5 Star reputation is not possible mathematically which ends up in 4.9999.

I'm attaching a .xlsx file just to make it more clear. In Table "Reviews Needed to Reach Goal" a green box for 5 Star Reviews should have a formula that calculates number of reviews needed for the required rating and the rating is being entered manually by user in CELL O,21 highlighted in Blue.

Help in this regard is much appreciated...

Thanks,
AISM