Formulae for accuracy ratings?

I'm trying to create a spreadsheet to help me audit some work, and I've done the basic parts but was wondering if I could take it further.

My spreadsheet at the moment works out accuracy percentages (number correct/number audited x 100). Is there a formula I could use to work out how many more audits are needed to hit the target of 80%? I've tried doing it as what 80% of the number audited woud be, take away however many has been audited now, but it shows me how many would've been needed for that target, how many more correct audits are needed in the future to hit the target.

Thanks

Wow, thank you so much, that's exactly what I wanted!