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Lecum said..
I've seen that if I put the usbox tracks just beside the end of the fin box, the rear footstrap would be just above the begining of the usbox tracks, so I think it's a good position, right?
Define "end of the fin box". If you do it like in the picture of the video you posted, that will work. I'd think that "end" refers to the tail of the board. That gives you a position similar to the Fanatic Stingray. If you start with a windsurfing board, that's quite far back, and you'll probably end up wishing it was more forward.
One reason to have some overlap with the fin box is that the back of the tracks can be attached to the reinforcements of the fin box. I'm not sure that's necessary, but I'd rather over-engineer than under-engineer.
If your board has a windsurf-typical narrow tail, you may find that you want to have your back foot placed in front of the straps, where the board is wider. You get more leverage over the foil this way, and better upwind angles. That's also an argument to put the track box more towards the nose of the board.