how do i make my beam do damage on someone


i want it so the beam does damage according to how much Str u have compared to the def the other guy has. Like if u have 100 str and he has 50 def u hit 50 damage on him if u have 200 str and he has 50 def u hit 150 damage on him

here are my vars

mob
var/Str = 5
var/Def = 5
var/maxHp = 100
var/Endurance = 5
var/Hp= 100
var/wealth=100
var/maxStr=5
var/Level=1
var/Rank="Starter"
var/inventory[0]


this is the hole beam code so just narrow it down if u need too



obj/projectile
icon = 'projectiles.dmi'
density = 1
var
delay = 2

length=20
dmg=2
splash
mob/owner
New(_loc,mob/_owner)

if(_owner)
src.dir = _owner.dir
if(!src.loc) src.loc = get_step(_owner,_owner.dir)
src.owner = _owner
Move()
src.length--
if(!src.length) del src
return ..()
Bump(atom/O)
if(ismob(O))

viewers(O) << "[src.owner] has hit [O:name] with \his [src]!"
else if(isobj(O)) del O
else if(isturf(O))

del src
Kurosaki_power
icon_state = "energyball"
delay = 1
dmg=100
Final_Strike
icon_state = "bigbang"
delay = 3
dmg=5
splash=5
beam
var
list/trails = new
max_trails = 0

New()
..()
src.icon_state = "[src.icon_state]head"

Del()
for(var/A in src.trails) del A
..()
Move()
var/turf/old_loc = src.loc
. = ..()
if(!.) return

var/obj/O = new(old_loc)
O.name = "trail"
O.dir = src.dir
O.icon = icon(src.icon,"[initial(src.icon_state)]trail")
src.trails += O
if(src.max_trails && src.trails.len > src.max_trails)
var/a = src.trails[1]
src.trails -= a
del a
Blue_Blast
icon_state = "ka_"
dmg=3
splash=2
EnergyBeam
icon_state = "energybeam_"
max_trails = 5
Blue_Blast1
icon_state = "poo"
dmg=3
splash=2
verb
Kurosaki_power()
set category = "Shoot"
var/obj/projectile/Kurosaki_power/P = new(null,usr)
walk(P,P.dir,P.delay)
Final_Strike()
set category = "Shoot"
var/obj/projectile/Final_Strike/P = new(null,usr)
walk(P,P.dir,P.delay)
Blue_Blast()
set category = "Shoot"
var/obj/projectile/beam/Blue_Blast/P = new(null,usr)
walk(P,P.dir,P.delay)
Blue_Blast1()
set category = "Shoot"
var/obj/projectile/beam/Blue_Blast1/P = new(null,usr)
walk(P,P.dir,P.delay)
EnergyBeam()
set category = "Shoot"
var/obj/projectile/beam/EnergyBeam/P = new(null,usr)
walk(P,P.dir,P.delay)