The positioning of the tray is important, but you have a bit of leeway. Plus, it's not too difficult since the edge of the tray is frozen to the short rail. It's off center though, so the second row of balls is even with the middle diamond, give or take.
You can think of this shot as a four rail kick shot into the bottom rail. I like to make sure my cue stick is over the corner pocket and I aim to the opposite long rail, deep in to the corner pocket. I just use a little top and no side spin, and then make adjustments with the cue ball starting point if I need to.
This is a neat little tray shot I picked up watching Robert Byrne's trick shot videos.
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