How it works
Your final grade is your current grade weighted by the part of the course already done, plus the final exam weighted by what it's worth. So the score you need is (goal − current × (1 − weight)) ÷ weight.
Example
With 82% so far, a goal of 85%, and a final worth 30%, you need (85 − 82 × 0.7) ÷ 0.3 = 92% on the final.
If the number is over 100
The goal can't be reached with the final alone unless there's extra credit. Try a lower goal, or ask whether other work can still be added.