formula to help with percentages

i am trying to work out a simple formula to help me determine how many more items i need to sell with an attachment to hit my preferred attachment percentage. i.e. the target is 36% which is located in cell G6. my colleague is currently on 31.6% which I input into cell G7. This is based on them having sold 19 items, which is input in cell C7. I know they need to sell 2 more attached items to get above the 36% target. but i would like a quick way of showing than for multiple colleagues.

Thanks in advance
Richard

Try this ...

=ROUNDUP(C7/G7*($G$6-G7),0)