You need to use a hole saw that has a centre guide drill, then it won't walk. It my shake a bit, but not enough to make the hole too big. didn't have a chance to find my notes today, will do it in the morning. Usually one side is nicer than the other, make that the inside and put your layout marks there. Use a thin felt tip as it will easily be removed.
Find the horizontal centre line, and the centre point. Make a new line 5mm above, and parallel (on the 550 it needs to sit slightly lower for frame rail clearence). I'll check this tomorrow but memory says....spacing is 70mm between 1 & 2 / 80mm between 2 & 3 / 70mm between 3 & 4. From the centre of the new line mark 40mm to each side, make a mark, and from there 70mm each way and another mark(s). These are your centre drill points for the holes.