I honestly do not know what you do not understand.
So if I have understood correctly, then there is no fixed number. For whatever reason.
It could most probably be because the pilot has an adjustable seat, or the glider has the Different
empty weight. The MIN and MAX numbers are already entered in table, so you need not to be expected.
As an an example:
GL = 280kp
min-490mm
max-545mm
So, difference - 55/2 = 27.5 (with MIN+MAX)
R=CoG
In these new documentaries the table has been extended even to 2 (330-340) further figures.
BR,
Stefan
p.s. see page-10 & 11...