Nostalrius Begins - Quality wow vanilla realm (1.12)

is this possible?, i want to make it so a certain button that when im low on mana will cast the highest flash heal possible for the amount of mana i have left

Lets say you have an table with the different mana costs of the spell:
local a = {200,400,600,800,1000}
for c,v in pairs(a) do if UnitMana("player")==v then CastSpellByName("Flash of Light (Rank "..c..")"); break end; if UnitMana("player")<v then CastSpellByName("Flash of Light (Rank "..(c-1)..")"); break end end

Something like that. I have not tested it though