My placement system won't stack. Any ideas?

So I have a placement system and I want my items to be stackable. I have everything correctly set up but when my mouse goes above the item, it of course goes into the sky and I don't know how I can fix that. This is the code:

	if stacking then
		posY = math.floor(mouse.Hit.Y) + step
		
		if posY > MaxHeight then
			posY = MaxHeight
		elseif posY < startingY then
			posY = startingY
		end
	end
end

Also, this is "stacking:

 local stacking