Randomize the size of an Explode

[daraku]

I've a problem i would randomize the size of an Explode within a range without use an Helper. I've used a weird formula, but it don't work

[State 0, Explod]
type = Explod
trigger1 = MoveContact || Pos Y >= 0
anim = 7003
ID = 7003
pos = 0,0
postype = p1
facing = 1
vfacing = 1
bindtime = 1
vel = 0,0
accel = 0,0
random = 0,0
removetime = -2
scale = 0.6+1/(random = [3,10]), 0.6
sprpriority = ifelse(random<500,2,4)

[Odb718]

try making a var to catch the random value. Then do the math based with the variable.