Work backwards from a target final grade.
Working backwards from a target inverts the weighted-average formula. Your graded work contributes its percentage scaled by the weight it covers; the target minus that contribution is what the final must supply, and dividing by the final's weight converts it into a score on the exam itself. The margin figure is the planning number that matters most: it tells you how much room a perfect final leaves above your target, and therefore how much marking variation the plan can absorb before it fails.
Score required for a target final grade
Required final = (target grade - grade so far x (1 - weight)) / weight
The target cannot be reached. The margin figure shows by how much a perfect final still falls short, so you can pick the highest grade that is actually attainable and plan to that instead.
No. Aim several points higher. Rounding at grade boundaries, one misread question and normal marking variation all erode a plan with zero margin.
Yes — add their weights together and treat the result as one block. The required score is then the average you need across all of them.