The formula
Needed on final = (Target − Current × (1 − Final weight)) ÷ Final weight.
Everything as decimals. Current is your average on the work done so far, weighted, out of the portion of the grade already assessed.
Example
You're at 72% on the 60% of the course done so far. The final is worth 40%. You want 75 overall.
Needed = (0.75 − 0.72 × 0.60) ÷ 0.40 = (0.75 − 0.432) ÷ 0.40 = 79.5%.
So a 79.5 on the final lands you a 75.
Do it with the calculator
Use the final grade calculator on this site. Put in your current average, the final's weight and your target. If it says you need over 100, you need a different target, and it's better to know that now.