I don't understand what you mean , you know what Set means?
If you mean the following:
-when a unit's health goes below a certain value: order unit to move back, distance equal to 500.
Use Pharaoh's trigger except change Unit1 to Triggering Unit in the second trigger.
I think that should work, but i'm not very experienced GUI'er
PS: Ignore best player 88, he's a troll