Formula Help // Refer to a Prompt from one Subassembly from another Subassembly

Hi There,

library V50.7

This has bugged my for a while and has stopped me from doing many paramatric things I wanna do.

How can I refer to a Prompt of one Subassembly from another Subassembly?

I have a bottom panel with a grommet hole in subassembly "Upper Carcase". The X location of the grommet hole is in the center of opening 1 ( bay 1) of another subassembly " Horizontal Splitter".

I tried (S!Opening_Width_1)/2 in the formula, but it doesn't recognize the name. Looking info the "Paste name" option, i can't even find the name " Opening_Width_1". My "Nested Subassembly Variables" in the "Paste name" is greyed out? Wondering is there anything to do with that? How can i load it up? What's the difference btw "Subassembly Variables" vs "Nested Subassembly Variables"?