Hi vannibombonato,
As above indicated, K88 total depth is 34.9 cm (from back-wall side to front side). If I undestood your inquiry, what you need to know is that the backside to the key placement is 18.4 cm and the keys to de fronside is 16.5 cm. You should also consider a free space between the backside and the wall of at least 4.5 cm for wiring staff (USB, MIDI, PSU, pedals). Of course, much more space is needed between K88 and the wall if you use the base for the laptop
Hope it helps,
elaledou