centering (another stupid grading thread)

[darwinbulldog]

I have a graded card that I want to submit for a crossover. I know the different grading companies list the grading criteria on their sites, but SGC at least gives just one number for centering (e.g., an SGC 84 has 70/30 or better centering) and PSA gives only two (one for the front and one for the back). There are, of course, four centering measurements (vertical and horizontal on both front and back). For "eye appeal" the front horizontal centering seems to be more important than the other three combined, but to what extent is that the same when actually getting a card graded? From a technical perspective it seems like they should all count equally, but PSA specifies otherwise in the front vs. back cutoffs and both SGC and PSA explicitly verbalize some wiggle room into their grading standards for something like eye appeal.

Does anyone know specifically how it works in practice? Let's say for an SGC 84, do all four measurements have to be 70/30 or better? Or just the main one but the other three have to be close? Or is it that the average of the four measurements has to be 70/30 or better? And is this a trade secret, or should I just call and ask them?