What velocity values should be used in Pianoteq to attain the following dynamic levels in a piano score? In case it depends on the model, I am interested in values for the YC5.

ppp
pp
p
mp
mf
f
ff
fff

the velocity graph in pianoteq gives you the standard values with the linear curve .Pianoteq manual indicates that the reference points by default are p=32  mf=64  f=96 .   But you can adjust these values of course and remap the values to include pppp and ffff dynamics  which  you find in some Rachmaninov scores . In which case you may reserve  pppp with any value between 2 and 8 ( velocity 1  triggers a  silent note in pianoteq)  and 127 for ffff which in Italian notation corresponds to the absolute maximum your keyboard can produce .

ppp =    8
pp   =  24
p     =  40
mp  =  56
mf   =  72
f      =  88
ff     = 104
fff    = 120

If you find this isn't rendering the way you'd like for a given piece, you can adjust Pianoteq's velocity curve to effectively shift all the velocities up/down, expand/reduce the range or change the slope in different ranges without having to re-visit the individual note velocities, understanding that the variation of dynamics within each marking's range is where all the magic happens (so far as velocity goes) in an actual performance.