Random spawning, spawning off platform?

I am trying to spawn these parts randomly across a bigger part. When I do it spawns them off the platform. Some are near / touching the platform but most are off it.

Here is the code

local amt = 10

_count = 0
while _count < amt do
    local model = script.Parent.Basic.Value:Clone()
    model.Parent = script.Parent
    model.PrimaryPart = model.Primary
    local DesiredCFrame = CFrame.new(math.random(0, script.Parent.Size.X)+script.Parent.Position.X, model.PrimaryPart.Position.Y+script.Parent.Position.Y, math.random(0, script.Parent.Size.Z)+script.Parent.Position.Z)
    model:SetPrimaryPartCFrame(DesiredCFrame)

    _count = _count + 1
end