A scanline in VESA 640×480 mode has larger portion reserved for active picture data (640/800 =80%) than DC scanline (640/858 =~75%) so both should need separate adjustments unless there’s an automatic size adjust (at least my Sony monitor from that era didn’t have one).
BTW, how did you end up with those [H, V] values? Assuming DC uses a fixed 27MHz pixel clock (divided by 2 for non-VGA), [H=855, V=526] would result to 60.04Hz refresh rate. Was your target to get closer to 60Hz than with standard (?) values [H=858, V=525] that should generate to 59.94Hz?