Did you ever get any answer to this? I have similar request, for similar reason. For example it would be nice during a tool change script to set the inactive toolhead temperature to its print temperature minus 10 degrees. Then if I modify the profile to a different print temperature, the tool change script would not need to be modified because it is in the form of a mathematical expression based off the print temperature.
The closest thing I can think of is defining extra temperature controllers, for example you can have "print temperature" and "inactive temperature" as two different temperature controllers on the Temperature page. You still have to remember to set these two controllers to be 10 degrees apart in the Temperature page, but at least you don't have to go editing Gcode. It is a slight improvement, but only slight.
I saw this thread on another forum. Interesting, there appears to be a "MATH" command, but nobody knows how it works.
https://www.reddit.com/r/3Dprinting/com ... ipting_in/