Plus, there are different levels of underrotation. Was the jump slightly underrotated or very clearly underrotated (such as the skater's entire blade being on the ice forward)? It doesn't matter according to CoP and what makes most people really infuriated is that most underrotations are slight and yet they receive a MASSIVE penalty when called.
There are different levels of penalty, but they don't scale evenly with the degree of underrotation.
Assuming that there are no other errors and the only error is the underrotation:
If the jump looks a little short to a judge and the tech panel does not downgrade it, the jump would get full base value and -1 GOE from that judge
If it looks a little short to a judge, or even if the judge thought it was clean in real time, and tech panel does downgrade it, it gets the base value of the lower-revolution jump and -1 GOE
That's the biggest jump in final value for the element. For practical purposes the rules have to draw the line somewhere and 90 degrees was the point decided on. The difference between 89 vs. 91 degrees, or let's say even 80 vs. 100, is not going to be perceived 100% consistently depending on viewing angle and other factors of the individual callers' or judges' perception, but the decision whether or not to downgrade a borderline jump can make several points worth of difference
If the jump is obviously, unquestionably somewhere between 90 and 180 degrees short, and downgraded, it would probably get -2 GOEs along with the lower base mark
If it's obviously a full 180 degrees short, or worse, it would get -3 along with the downgrade
If there are additional errors (e.g., touchdown or hop on the landing) in addition to an obvious downgradable jump, the GOEs would also probably be -3
If there are more, or more serious, additional errors (e.g., step out, or problems with other phases of the jump before the landing), the GOE would still be -3, and unless there's a fall the GOE would end up being the same for this badly flawed cheated jump as for a jump whose only flaw was being cheated -- at a certain point the compounded problems with the element stop being reflected with worse and worse scores -- this was also true for short program deductions in the old system
Then if there's also a fall, the jump would be downgraded, the GOE would be -3, and there would also be a -1.00 fall deduction