Thanks a lot for the explanations, as my algebra lessons are (too much) far away !
I understand there is an average of the values of probes with a Least Squares method ; the BLCM is still 3x3. So, can we consider that an unflat bed (like the one with carbon fiber) is considered as a flat/rigid bed, no matter what value you use for n ?

In addition, I've seen an another great evolution for your already (very) great G29 enhancement ; instead of probing the full bed size, it is possible to probe just the printing area. It is very usefull, because you reduce the probing zone, reduce time of probing, improve the accuracy, ... ; a guy has already made this on github (https://github.com/ErikZalm/Marlin/pull/1022) ; 3 files have been changed. Is there a way to merge the code ?
Thanks