Try this (in your fightclass):
In spell name put:
local name = GetSpellInfo(116694);
RunMacroText("/cast " .. name);
(change 116694 by your spell id)
In "Spell Settings":
"Timer" > Put cast time (in milliseconds)
"Can move during cast" > No
"Check if know spell, is usable and good distance" > False
"Check if target is in view" > False
"Combat only" > False
"Not spell, is lua script" > True
In "Spell Conditions":
Add "Me In Combat" > value "False"
Add "Health Percent" > type "SmallerOrEqual" and put at "value" the same value of your current setting "Food on x %" (80 by default).
ps: If you have read before post, don't forget to restaure default value at "Food on" in advanced general settings tab "Food/Drink".