How to put part on top of other part/surface?

RS.RenderStepped:Connect(function()
	if toggle == true then
		part.Position = Vector3.new(mouse.Hit.Position.X,mouse.Target.Size.Y+part.Size.Y/2-part.Size.Y,mouse.Hit.Position.Z)
	end
end)

I'm doing some tests to enchant my positioning and cframing, today I tried to do some kind of part placement that places on top of other part but my tries wasn't succesful, it either places on center of part, or places it wrong. I tried this also.

RS.RenderStepped:Connect(function()
	if toggle == true then
		part.Position = Vector3.new(mouse.Hit.Position.X,mouse.Target.Size.Y/2+part.Size.Y-part.Size.Y/2,mouse.Hit.Position.Z)
	end
end)