Navigate to
c:/programfiles(x86)/steam/steamapps/common/legendaryheroes/data/english/corespells.xml
Open the file with Notepad, or better yet Notepad++ (free download).
Look for the block of code with the SpellDef "FirstAid"
In that SpellDef you'll see the chunk of code I copy-pasted in my first reply.
Replace the selection I highlighted in pink with [Calc]
Voila! Your very first mod.