How to refer to a substitution variable inside a mapping SQL ?

Summary: Hi, need your help to direct me in how do I refer to a Substitution Variable named Total_New_Assets inside an SQL mapping, as follows:

I have a mapping that is working well but I'd like to replace the number 100 by a Substitution Variable named "Total_New_Assets".

This the current mapping that works fine:

It currently does the following:

This is my Subvar:

And I'd like it the mapping to be possible to be changed manually to get this result for example:

I've tried changing the 100 as follows: 'I' || TRIM(CAST(CAST(TRIM(UD17) AS INTEGER) + $Total_New_Assets$