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Mark the jug just a bit longer
Notice that I've marked the jug at 2.75".
You need that extra space - unless you're only taking pictures of the surface of large, flat objects.
Whatever I put under this thing is going to have some height to it and if I cut my jug to the exact length then I probably wouldn't be able to focus on it. By leaving myself some space I allow for my subject to have some thickness to it.
How much space you leave isn't critical, but it does matter. Too little and you'll find yourself unable to focus on things. Too much and you're 'wasting' some of your image because you'll have to crop the image too much to 'zoom in' on your subject.
Try to guess about how far up from the surface the top of your subject is going to be and allow just over that. If you'll be using this to take pictures of work in progress on your bench don't forget to allow for any distance between whatever the bottom of the jug will rest against and the top of your piece.