This is the result of numerical investigation filtered by the "TAR"* method. For sure your marvelous onboard computer knows better...
The objective is to provide simple figures, easy to memorize, per family of glider (Club class/15m/18m/20m/unlimited), to be used with "Standart Mc Cready ring" (like the good old time).
Example : today I fill my LS6 with 80L of water. I should consider increasing by 0.4m/s my McCready setting due this additional wing loading - on top of other setting (80/100*0.5m/s=0.4m/s)
Don't hesitate to use this "enhanced rule of thumb" !
(& keep in mind it is always more efficient to have a McCready setting "a bit too less" than "a bit too much"...)
* "TAR"="That's About Right" method - i.e. there is a certain dose of approximation in all this.