How the calculation works
A course grade is a weighted average. If the final is worth 30%, everything before it is worth the other 70%, so your course grade is your current grade times 0.7 plus your final score times 0.3. The calculator runs that equation backwards: take the grade you want, subtract what you've already banked, and divide by the final's weight.
Say you have 84% and the final is worth 30%. You've banked 84 × 0.7 = 58.8 points. For an 85 overall you need 26.2 more, and 26.2 ÷ 0.3 is 87.3% on the final. For a 90 you'd need 31.2 more points, which is 104% - more than the exam can give you.
Two things trip people up. First, "current grade" has to be the grade on everything except the final; if your portal already counts the final as a zero, it will look far lower than it really is. Second, some courses weight categories rather than single exams. If yours does, work out the current grade from the categories first, which is what the helper beside the calculator is for.
When the answer is over 100%
A required score above 100% means the final alone can't get you to that grade, but it isn't always the end of the story. Read the syllabus for anything that changes the arithmetic: a final that replaces your lowest midterm, dropped quizzes, extra-credit work, or a curve. Any of those can move the number, and it's worth asking your instructor directly which ones apply.
Otherwise, the useful move is to look one grade down. The table shows what every letter needs, so you can see whether the next one is comfortable, tight or already secured - and plan your revision for the target you can actually hit, instead of spending the week anxious about one you can't.
Getting the score you need
Once you know the number, the question is how to study for it. The research is fairly consistent here. Testing yourself beats re-reading: in Roediger and Karpicke's 2006 experiments, students who practiced recalling a passage remembered much more of it a week later than students who spent the same time re-reading it. Dunlosky and colleagues' 2013 review of ten common study techniques rated practice testing and spaced practice the most useful of the ten.
In practice, that means turning your notes into questions and answering them on several days, not in one long session the night before. Sit at least one full practice exam under timed conditions, then spend most of the remaining time on the questions you got wrong. There's a longer walk-through in how to build a practice test from your lecture slides, and the study schedule maker will lay the days out for you.