Yep, "around' is the best you can hope for.
Neither wind direction nor speed is constant during the flight of the ball.
Few tail winds are exactly on the line of the shot.
An 18 mph tail wind that averages 45° to the line of flight of the ball during the flight of the ball acts like a 12.5 mph wind.
Another factor is the % of ball spin.
It's hard to tell where exactly on the ball 50% back spin is. I've gotten much more careful when using top or back spin on the ball since I realized that very small changes to the spin can have a pretty big effect .
Another observation is that the effect varies depending on the loft of the club.