Return mana percentage in combat or if less than 100%

gistfile1.lua

function()
    local mana = math.max(0, UnitMana("player"));
    local max = math.max(1, UnitManaMax("player"));
    local p = math.floor(mana / max * 100)
    if UnitAffectingCombat("player") or p < 100 then
        return string.format("%.f", p);
    else
        return ""
    end
end