means dish indeed.

Anyway, assiette of calcul is like our "assetto di calcolo", and here assetto is - ah, very hard to describe.
Assetto comes clearly from "sedere" = to seat, and the meaning is that you as a person are in assetto di calcolo when you seat comfortably, and ready to computate, and in the appropriate mood to do it.

Something like "mode" in the watch instructions.