Exactly! It's <ctrlSetText> and not <lbSetData>.
1-GetDammage player - Health of the player
GetDammage player - Damage of the player
So, in your script:
ctrlSetText [101, format["%1",1-(GetDammage player)]]
where '101' is the idc of the class you want to edit.
Got it? If not, keep trying!