How the final grade calculator works
Your overall grade is a weighted average of work completed and the final exam. Everything except the final makes up (100% minus the final's weight) of your grade and is already determined by your current standing. The calculator solves the weighted-average equation for the one unknown, the final score, so you know exactly what you need.
Setting a realistic target
If the required score is very high, consider whether a slightly lower target grade is still acceptable, and check whether extra credit is available. The lighter the final's weight, the harder it is to move your overall grade with it, in either direction.