I would imagine that the designer of the pro originally set the cg at the 77 mm mark. This point is about as far back as the cg can go and still have a controllable sailplane. I would also assume this is the max L/D point. This is where the factory places the recommended battery.Then after some testing and out of consideratrion for the newer pilots the cg was suggested at 70 mm. This gives a more contollable sailplane, but it requires moving the factory battery fwd until its almost out of its holder or add weight or get a heavier battery. That said the sailplane flies best at 77 mm. At that pount elevator trim should be almost zero. If its at zero the elevator creats less drag because its not offering much resistance to the horizontal airflow (drag).
Now if you move the cg forward You have to change the trim to pick the nose up or else fly faster both increases drag
. A sailplanes in general terms has max L/D just above stall speed.
Now the Pro is light and doesnt penetrate well. There are a lot of people here and other threads that state on windy days " I move the battery forward (move the cg fwd)". This will help pentration but you will not stay up as long due to drag and if you do not change the trims it will not stay up as long either. The solution is to add weight as near the cg as possible. Some have the right idea by putting steel rods inside the wing tube. ASdding weight here does not require changing trims. The plane will fly as well with the weights as without and you greatly increase penetration. Weight here does not change the L/D (up to a point) Generally the higher the wind speed the more weight that needs to be added. L/D refers to how far forward a plane will fly for a given drop in altitude eg. 30:1 L/D means for every foot it drops in altitude it will fly 30 feet forward. Best L/D is the speed at which this 30:1 ratio occurs. Faster or slower than that speed will decrease this ratio. On purpose or not we try to fly at best L/D. (even in wind). How fast a sailplane flies is up to the pilot. If the wind is faster than the forward speed of the aircraft we have to fly faster by putting the nose down. We pay for this with a smaller L/D which means we are on the ground faster. If the wind is not faster than the best L/D speed then we can still fly the plane with at best L/D (dont forget to add glider speed to the windspeed)
See this link for non calculus explanation of how this stuff works.