i find that center-to-shoulder chainline is a useful number if i'm trying to avoid measuring things. i add that number to the chainline of the sprocket if i want to calculate a chainline without measuring anything. center-to-shoulder distance is very hard to measure, even with calipers so here's what you should do.
measure distance from flange center to flange center and divide by two. then measure the distance from the center of the drive-side flange to the center of the teeth on the cog. add those two and you've got your total chainline (no need for sheldon's charts).
EDIT: ignore all that. locust's got you covered