I haven’t worked as a grader since 1998 and won’t try to speak for others. As for myself, however...often, I didn’t use magnification, other times I used 5x and on rare occasions, stronger. My decisions weren’t typically based on any particular contemplated grades. But rather, they were based on factors such as the size of the coin, whether the surfaces were noticeably toned, when I was having difficulty determining whether a flaw was man-made vs. mint-made and potential value of the coin. Again, nothing approaching a formula.