What do I need on the final?
Type three percentages. The answer appears as you type. Nothing you type leaves your browser.
How it works
The final counts for a share of the course grade, say 25%. Everything before it counts for the rest, 75%. Your current grade is the grade on that rest, so it already brings some points with it: 84.5 x 75 / 100 = 63.375 points.
To finish on 90 you need 90 - 63.375 = 26.625 more points, and the final is worth 25 points at most, so the score you need is 26.625 / 25 x 100 = 106.5%. The page shows that line by line for your numbers.
The page uses exact arithmetic, with no floating point. The answer is rounded up to the next hundredth of a percent, so reaching the number shown is always enough.
What the notes mean
Over 100 means the target is out of reach unless your class offers extra credit. "Already there" means that even a 0 on the final leaves you at or above the target.
What it asks for
Percentages from 0 to 200, with a dot for decimals and up to 4 digits after it. The final's weight is above 0 and up to 100. A comma such as 1,5 is refused instead of being read as 15.
What it does not cover
It assumes your current grade is the percentage on everything except the final, and that the weights come from your syllabus. If your class grades in points, turn the points into percentages first. It does not know about dropped scores, curves or rounding rules at your school.