How can I determine the amount of trim drag? I fly an ASW27, which is a 15 meter glider with flaps.  I am flying with the CG at 62% aft.  I use down flaps while in a thermal and climbing; negative (up flaps) are used in cruise between thermals.  typical thermals speed is 50 to 60 kts (with ballast) and 90 to 100 kts in cruise.  This glider has a T tail and I need nose up elevator for thermal and nose down elevator for the cruise.  

I notice that in cruise, the elevator is deflected down about 10 deg.  Is that deflection creating a significant amount of drag that could reduce my glide performance?  I have the ability to move the CG by dumping the ballast in the tail tank
As I mentioned in my comments. You could test this empirically with the experiment described there.  If you find the additional ballast improved the glide performance of the craft, is there an aftermarket STC kit available which installs new ballast tanks in a more optimum position for a more neutral cruise trim?

I am a little concerned with the idea of continuing to move the CG aft of its current position in order to optimize the performance at the expense of. Asking the craft less stable about the lateral axis.  No increase in performance is worth your life.