You know what, you told the truth...it is not accurate 100% but after exam day done you may remember problems that were easy and you solved but did not count them in the first day "the exam day I mean" and find out that you were pessimistic because you under pressure of marathonic exam restricted by time..Because I don't think it can be accurate. I think one's memory would be most accurate and most correctly able to gauge the # of "feel-good" problems during the exam (and even that is subject to human error). But trying to remember the number of "feel-good" problems after you leave the exam room? Mmmmmm..... I don't think so.