The formula
needed = (target − banked) ÷ weight still to be assessed
Where it comes from
What you have already done is counted towards the final grade in the proportion it was worth, so what is missing is not "the grade you want" but what you have to score across the remaining weight to get there. The arithmetic is simple and the surprise usually runs one way or the other: sometimes far less is enough than you feared, and sometimes even a perfect score no longer does it.
How to work it out by hand
- Multiply each grade you already have by its weight and add them up: that is what you have banked
- Subtract the banked total from your target
- Divide by the weight still to be assessed
- If the result is above the top grade, it is no longer reachable
What is worth knowing
The two figures at the extremes are worth a look as well: the most you can reach with a perfect score on what is left, and the least you would end up with even handing in a blank sheet. With 60 % already assessed, those two figures are only four points apart, which explains why halfway through a course the final grade is far more settled than it looks. And watch the weights: if the course requires a minimum score on the final exam before anything is averaged, this calculation does not know about it and you have to check the syllabus.