I gues this is where the confusion is. In your example, you indeed have 10 knots tail wind in the direction you heading. You assumed the wind is 90 degrees, but it is not since you drifting. Now if instead you crab to maintain heading, the tail wind component will change to head wind. So this is proving again that my formula is correct. This is the true head wind/ tail wind component as far as the glider see it, which is ultimately what impact your ground speed and glide perfromance over the terrain.

Ramy