why can't i snap to grid accurately

hello, i've seen a few threads about this from years ago and it seemed to be a real issue with a lot of people that, even with smart guides turned on things never seem to snap 100% accurately. I use a rectangular grid (not the default one, a custom made one) have snap to point/grid /glyph unchecked, and smart guides on. First of all, is there a setting to snap things to a custom grid?; rather than documnet grid, secondly, i've tried a few different smart guides preferences, but can never seem to stop smart guides from trying to snap to all the different shapes i've already made, rather than the custom grid, and thirdly even when i'm lucky enough to actually gert it snapping to the grid (which is rare) if i zoom in it's often not completely snapped, it's really frustrating, sometimes it's only by a few px, but pretty disconcerting knowing that it's probably the case for all of my anchor points, any help much appreciated