You can mark your line with a sharpie at 39 feet for a 9 ft rod. Then you will have about 30 feet our of the guides when the mark is just off the reel.

Some fly line/rod combinations have a "sweet spot" that seems to produce the easiest cast . This length can differ for different casters. If this happens with your outfit, you can mark that spot with a sharpie or whip a bit of tying thread on the fly line and cover it with some Uv cured resin so you can feel it with your line hand when that amount of line is off the reel.