Equation editor and external reference

Hi,

I am trying to define a dimension of a part2 with the geometry of the part1.

I found this thread on the forum, which gives some clues,

https://forum.solidworks.com/thread/21920

but how and where I can define the equation "D1@Sketch1@Part5<1>.Part"       = "D2@Sketch1@Part4<1>.Part" - 1

In the part5 ? in the asm1 ?

I guess that it is in the asm1, but how can I get any D1@Sketch1@Part5<1>.Part, without entering this beautiful name by hand ?

or for SW conceptors, to memorize parameters full of wise and to type them is the doom for SW users ?

SolidworksSketching