how to write edit time only logic ?

I am trying to make a tool to layout game objects on a grid at edit time. Grid info, such as grid coordinates of elements, aren't relevant at run-time because it should have been translated into world space coordinates at edit time.

I do not understand where i should put logic that will keep track of grid related data and convert at edit time grid coordinates into world coordinates. I added on each gameobject a custom inspector where i can edit grid-wise coordinates, modifying these properties should trigger translation from grid coord to world coord.