Migs...
I think that you will find that the permanent markers ware off quickly and tend to mark other sections of your line. A better method is to take some backing (preferably Gelspun) and make an eight turn nail knot at the 39 foot mark on your line. You will feel the nail knot when it hits your finger and it will mean that you have 30 feet of line past your rod tip if you are using the standard 9 foot rod. You will have the prober weight of the line for that weight rod past the tip to load the rod properly.