Lua [spell] Formula that uses a storage!

[sebas182]

local combat = createCombatObject()
setCombatParam(combat, COMBAT_PARAM_TYPE, COMBAT_PHYSICALDAMAGE)
setCombatParam(combat, COMBAT_PARAM_EFFECT, CONST_ME_HITAREA)
setCombatParam(combat, COMBAT_PARAM_USECHARGES, true)

local area = createCombatArea(AREA_SWEEP1)
setCombatArea(combat, area)

function onGetFormulaValues(cid, level, skill, attack, factor)
	local skillTotal, levelTotal = ((skill*1.1) + attack)/2, level/5/2
	return -(skillTotal + levelTotal), - (skillTotal + levelTotal)*2
end

setCombatCallback(combat, CALLBACK_PARAM_SKILLVALUE, "onGetFormulaValues")

function onCastSpell(cid, var)
	return doCombat(cid, combat, var)
end

Look this! It's an edited formula from Exori, that I modified for a weaker knight (warrior in my server) spell...
Now I want to use the same to make a spell that uses some values... The formula is:

local firelv = getPlayerStorageValue(cid, 24012)

((firelv * 1.8) + (magiclevel * 1.8))/2 + 1 -- between this min
((firelv * 1.8) + (magiclevel * 1.8))/2 + 15 -- and this max

In sample, firelv == 10 and magiclevel == 10 so:
((10*1.8) + (10*1.8))/2 + 1
(36)/2 + 1
18 + 1
19

((18) + (18))/2 + 15
(36)/2 + 15
18 + 15
33

Damage between 19 to 33.

I really need it, please help me!
Things like graphics and type I already know to modify, just need help in the FORMULA. PLEASE!

 

[Zisly]

Like this maybe?

local combat = createCombatObject()
setCombatParam(combat, COMBAT_PARAM_TYPE, COMBAT_PHYSICALDAMAGE)
setCombatParam(combat, COMBAT_PARAM_EFFECT, CONST_ME_HITAREA)
setCombatParam(combat, COMBAT_PARAM_USECHARGES, true)
 
local area = createCombatArea(AREA_SWEEP1)
setCombatArea(combat, area)


function onGetFormulaValues(cid, level, skill, attack, factor)
	local firelv = getPlayerStorageValue(cid, 24012)
        local magiclevel = -- no idea what the function is called :/
	local _min = ((firelv * 1.8) + (magiclevel * 1.8))/2 + 1
	local _max = _min + 14

	return -_min, -_max
end
 
setCombatCallback(combat, CALLBACK_PARAM_SKILLVALUE, "onGetFormulaValues")
function onCastSpell(cid, var)
	return doCombat(cid, combat, var)
end