The air mass in an anticyclone generally sinks over its whole area. That's what generates the wind which turns clockwise around an anticyclone (northern hemisphere).

I agree that just for programming the polar into the flight computer, the actual precision does not matter much.

As to the Idaflieg measurements - there is no secrecy involved. Manufacturers put their gliders at disposition for those measurements, and no data will be published in the first two years. This stems from the times of Nimbus 3 vs ASW22 - with people looking only at best L/D, one of the two gliders being published with 2 points more would have made the other manufacturer go bankrupt.

The data is not published anywhere. If you're interested in a measurement of your type of glider, you send Idaflieg a request by email. If it exists, they will send you a paper copy, and a bill of about 10-15 Euros.