Great video, Darren. Youtube won’t let me comment or I would have there.
You brought up the subject of locking the stock. When you have a known piece of stock, you enter the dimensions and lock them, as you did. Then you can size your geometry and position within the stock. When you specify one margin, the opposite side will adjust automatically.
But when you have a fixed geometry, you can enter desired margins all around (or one margin and check “centered”) and MeshCAM will tell you the stock size you need to give those margins. That is the case where you would leave the stock unlocked.