How do you set the position of the top of a part?

I'm trying to make a placement system, however I'm not sure how to place the top of the part on the top of another part. I want it placed on top because if it placed the middle somewhere then there would be a huge block coming out of the ground. Here is my script which works, but it places the middle of the part instead of the top.

local Players = game:GetService("Players")
local UserInputService = game:GetService("UserInputService")
local Player = Players.LocalPlayer
local Mouse = Player:GetMouse()
local Click = script.Parent

Click.MouseButton1Down:Connect(function()
	local Placed = false
	local Cancelled = false
	Mouse.Button1Down:Connect(function()
		Placed = true
	end)
	UserInputService.InputBegan:Connect(function(input)
		if input.KeyCode == Enum.KeyCode.Q then
			Cancelled = true
		end
	end)
	local Baseplate = Instance.new("Part")
	Baseplate.Size = Vector3.new(150, 10, 150)
	Baseplate.Anchored = true
	Baseplate.Parent = workspace
	repeat
		wait()
		if Mouse.TargetFilter ~= Baseplate then
			Mouse.TargetFilter = Baseplate
		end
		local Character = Player.Character
		local MousePosition = Mouse.Hit.p
		if Mouse.Target ~= Baseplate and Mouse.Target then
			if Mouse.Target:IsA("Terrain") then
				Baseplate.CFrame = CFrame.new(MousePosition.X, Mouse.Target.Size.Y - Mouse.Target.Size.Y*0.9, MousePosition.Z)
			end
		end
	until Placed == true or Cancelled == true
	if Cancelled == true then
		Baseplate:Destroy()
	elseif Placed == true then
		script.Parent.Parent.Parent:Destroy()
	end
end)

if the 2 parts never rotate, then its simple arithmetic

local PartToSpawn = Instance.new("Part", workspace)
--modify part properties and whatnot

local Part2 = workspace.Despacito -- part to spawn ontop of

local Y = Part2.Position.Y + (Part2.Size.Y/2) + (PartToSpawn.Size.Y/2)-- final result

if only the part to teleport to is rotating is a little more complicated

local PartToSpawn = Instance.new("Part", workspace)
--modify part properties and whatnot

local Part2 = workspace.Despacito -- part to spawn ontop of

local CF = Part2.CFrame * (Part2.CFrame.topVector * Part2.Size.Y)-- final result

if both rotate its too complicated for me :\