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)