I've been fitted, and have the measurements from that session. They turned out to be extremely close to what I had already determined I needed, by shooting at a plate and making dimensional adjustments myself. That said, the only thing knowing my measurements are useful for is when looking at guns online, or ordering a gun. I can usually know, within a range, if a gun is unsuitable for me by examining the dims on it. But, nothing can take the place of shouldering the gun a few times. I can mount a gun and know immediately if I will be able to shoot it reasonably well. Years of shooting different guns is worth a lot.
