Some years ago I did a gliding instructor course here in Germany which involved a lot of spinning. It was interesting that with a rather large instructor trainer in the front that one of the two types of gliders would not stay in a spin, we were using K13 and K21s. We did days and days of spinning and the result was always the same on the K13 half a turn and despite full crossed controls it was out. So my practical experience backs up the CG theory at least on some machines. With light trainees I have found the K13 spins to your heart’s content.