If it is, can I make a list, and copy/paste this into every template I want to sew, so that I get a much more accurate pattern? 

Is there a tutorial for this?

Thank you for your help.

The sz variables cannot be overwritten. There are two options:

You can create a new variable as in underbust=sz17+0.5

OR

edit the saved size in PRO mode and fix the sz17 as you see fit. Please make sure to double check the other measurements of this size in this case, as they will not be recalculated to match the new sz17 automatically.

Hope this helps!