This was a real stinker. Not in any dictionary I could find, English or French. But I found several
sites that mentioned it. Many were names of reataurants. One was a magazine "L'Assiette de la Beurre"
A couple were about tables for computation. An "assiete de calcul" was a hand held calculator. Finally
I found a couple sites that showed pictures of"Assiettes" that were plainly dinner plates.
I'm still annoyed.

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.