Ehhhh...that can work, but.........doesn't help to determine CG envelope when your butt is in the glider. Static on the ground (done properly) tells you where you are before getting in.

-Centered CG is nice.
-Forward CG can be good for training/low timers. If you don't pay attention to pitch and relax on the stick the glider will speed up, staying away from stall.
-For XC, sit on the ground, mark the stick where the elevator is neutral to the stab, fly, see where the stick is flying a constant speed used for XC, change weight (CG) until the glider flies hands off at the required speed (stick relative to mark made before on neutral controls). This is usually an aft CG.

Yes, I have accidentally flown a CG out the backside before....once (ASW-20). Over the top spin entries when doing zoomies into thermals told me something was not right. The glider owner (flying tow at that time and saw a couple spins) asked over the radio, "spin much Chuck?!?!".