First of all, make sure the marker is set properly. Go out in the field and set one down then the other or both down, if your hydraulics allow. There is an extension adjustment on every marker I've seen.
In the US, almost every marker on a planter or drill is intended to put a mark which should be followed with the hood pointer right down the center of the tractor. In Europe the markers are often shorter and intended to place a mark for one of the front tires to run in. This is possible on smaller, open tractors but rarely done in the US. Following is based on US equipment which guides the center of the tractor on the marker mark.
Take a tape measure and make a centerline mark on the frame of the planter.
Calculate the overall planting width = number of rows times the row width. For example 6 rows x 30" row spacing = 180" = 15 ft exactly.
Measure from the centerline on your planter frame out each side to the overall planting width (180" or 15 ft in the example above). THIS is where your marker end disc should be set! Do this for both sides.
On a drill you do the same thing except the row spacing is usually 7.5" or 10".... Is this what you are asking for?
The trick is to keep yourself in the center of the seat when sighting over the hood ornament. Steer the tractor so the hood pointer stays lined up on the mark made on the previous pass.
Good luck. Jim
edit: most markers also have an angle adjustment to try to make the mark dark enough/move enough soil so you can see the mark on the next pass. IF you cant see the mark then try to pivot the marker end disk to make it more aggressive.