Just a word of warning to others who may try to duplicate this (DAMHIK): Pay very close attention to where the TV will end up after the bracket is installed.
We bought a TV that we sized to fit a cabinet/shelf just like that. We wanted the largest screen that could fit in the rectangular hole. We found the perfect TV, one with less than 1/8 inch clearance on each side. I measured everything very very carefully. Then I measured it very very carefully again. I was careful to include the correct left/right offset in my measurements, and then I installed the bracket.
I forgot "sag." The weight of the TV caused the arm to sag down slightly. The bottom of the TV bumped into its lower shelf, instead of being nicely centered and framed in the cabinet. I measured the sag offset, then reinstalled the bracket that much higher, and everything fit perfectly.
For something like that, it almost pays to install it first on a piece of scrap plywood, then measure. It is too hard to predict to those fine tolerances.