I might be misunderstanding the question, but if you're trying to adjust the pump's relief pressure you'd have to stop the water from flowing out through other places somehow or it won't be able to build any more pressure than what it takes to run water through the machine. If you block the group head with a blind filter then you can set the relief valve on the pump and know at what upper limit it will open. Even still, the pressure you brew at will be determined by the resistance to flow created by the grounds. Only when the grounds are fine enough and well enough tamped to prevent the water from flowing through will the pump-relief be relieving a significant amount of water.
If you adjust pressure against a blind filter (with no flow) then your indicated pressure will probably read about one bar high. So you adjust to say, 9 bar to get about 8 bar on the puck during an actual shot. That will get you pretty close to what most people consider "ideal."
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