Use the remaining weight, not raw points
The needed score depends on how much of the final grade remains. A target can be easy with 50% remaining and impossible with 5% remaining.
Feasibility matters
The tool reports when the required score is above 100% or below 0%, which means the target is impossible or already secured under the simple weighting model.
Account for syllabus exceptions
Extra credit, grade replacement and curved exams can change the result. Use the syllabus rules when they differ from a straight weighted model.
Plan more than one target
Compare several target grades rather than focusing on a single threshold.