This is going to depend on a lot of things. For the power MOSFET,
you're really talking Cdb as the dominant term because B=S by
close-in strapping. Then it's raw junction depletion capacitance
with Vds dependence (when off) or a pretty good D-S short and
not much use in figuring C, when on.
Short channel FDSOI MOSFETs, it's really the metallization and
S,D region fringing fields across the short L gap if the buried oxide
is thick. That's a nasty thing to try closed form analysis on;
TCAD with good EM tools is the best hope for accuracy in my
opinion.