See how the final exam weight changes your grade.
The final's weight decides how much of your grade is still moveable. Whatever weight the final does not carry is already locked: your current grade times the remaining fraction. Add the final's contribution and you have the projection. The floor and ceiling bracket every outcome still possible — and the gap between them equals the final's weight exactly. A 30% final can move your grade by at most 30 points, which is why the same exam score is decisive in one course and cosmetic in another.
Final exam impact on the course grade
Course grade = current grade x (1 - weight) + final score x weight
By its weight and no more. The swing figure is the arithmetic limit: a 25% final moves a grade by at most 25 points, and only if you go from a zero to a perfect score.
Run the calculator twice. First with the final at its stated weight, then with the replaced test's weight folded into the final's, and take whichever projection is higher — that is what the replacement rule gives you.
The floor assumes a zero, which usually means a missed exam rather than a bad one. It is most useful as the answer to whether you can still pass in a genuinely bad scenario.