The final counts for a share of your course grade and your current grade covers the rest. Needed score = (goal - current x (1 - weight)) / weight. For example, with an 82% now, a 20% final and a goal of 90%, you need (90 - 82 x 0.8) / 0.2 = 122% on the final, which is not realistic without extra credit; a goal of 85% would need 97%.
Enter 90 as the goal (or whatever cut-off your class uses for an A) with your current grade and the weight of the final. The result is the exact score to aim for.
Then the goal is out of reach unless your teacher offers extra credit. Lower the goal grade to see what is possible.
Enter 50 as the weight. The heavier the final, the more it can lift or drop your grade.
See also: GPA Calculator · Average Calculator · Percentage Calculator