I use the parallel guides attached to my holey rail. Once that first line of holes is setup, I bring up the stops on the parallel guides to contact the back of the panel. You are then setup to clamp in place and drill your first row of holes. After completing the first row, unclamp your panel and slide it forward, away from the parallel guide stops. Insert a spacer of the distance you wish the next row of holes to be offset from the first row, slide the panel tight to the spacer, reclamation and drill. I have spacers pre-made up in 32mm increments so I can easily set holes for installation of various drawer slides. If you need additional rows, just insert a spacer, or combination of spacers, for whatever offset you need. When that panel is completed, just slide it out, slide in the next, and you’re already set to drill your next panel.
I saw this method described/shown for use of the parallel guide setup, (think it was by Brice Burrell), and most likely saw the use of the spacers for positioning, as well.
A great method for doing multiple panels and it will be as accurate as you make your spacers.
Works best if the panel length is an increment of 32mm so the parallel guides are tight to your panel. If your panel is not an increment of 32, it will still work as long as you don’t have to shift your panel beyond the point that the parallel guide stops contact your workpiece.