No matter what someone else set their relief valve at, use what your manual says.
There may be other reasons why they set the pressure to 2611 psi.
Some people try and tweak it higher for more power, but it is hard to know how far to go up on the pressure relief and still be safe.
As far as where to put the gage, if you plug into a work port as dfkrug says, you will only read the pressure for that valve work ports.
If you can find a place to add a tee after the pump but before the first valve, that would be the place to add a pressure gage for the total system. If your hyd system only has banjo fittings, it will be hard to mount a gage permanently. It can be done by cutting and using a compression tee fitting for the gage.
So if you install the gage after the pump, which ever valve lever you operate, you will read the pressure on that circuit, be it the 3pt, remote valve, or loader valve.
The pressure gage after the pump in series with the hyd flow, will let you monitor and trouble shoot the hyd system.
If you only plug the gage in a work ports, you will read the pressure the cyl is developing.
If you raise the loader empty, you might only read about 500 psi to 800 psi.