Simple... attach the marker to the tip of your antenna. Then hover over your head and mark the bottom of the blades!

Nah, I usually spin them up till its just getting light so they are making some lift. I should have said that in the original post. If this isnt enough I guess you could stick a board through the skids or some other type of weight.

There is a safety issue of standing over a running helicopter that is light on the skids. One bump of the stick and you're mincemeat. Or a destructive tipover right in front of you. For this reason I would suggest an assistant use the marker.