Plan score improvements for a target grade.
Improving a grade means working backwards from the target. The work already graded has banked a fixed number of final-grade points — your current percentage times the weight it covers. Subtract those from the target and you have the points the remaining work must supply; divide by the weight still open and you have the average score you need on it. The smaller the remaining weight, the higher that average has to be, which is why the same twelve-point gap is trivial in week three and impossible in week twelve.
Score needed to hit a target grade
Required average = (target % - current % x completed weight) / remaining weight x 100
Because the banked points plus a perfect score on everything left still fall short of your target. That is the arithmetic telling you the target is no longer attainable — pick a reachable one and re-run the plan.
Add the syllabus weights of every item already returned to you. If two 10% quizzes and a 40% midterm are graded, the completed weight is 60%.
Plan slightly above. Rounding rules, a single bad day and marking variation all cut into a plan built with no margin, so aim two or three points past the boundary you actually need.