You'd want to start at about 600 psi or so. If you have any upgrades(valve, bolt, etc) it would run lower.
Some spysers may not run that low at first. Most of the ones I have dealt with needed anywhere from 650 to 800 but can be brought down to 600 easily with home mods.
Just play with it and set it to where the gun fires well at a good velocity.
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Current guns
Blue DM5
Blue to Black Lotus Cocker(in the process of building)
Black 2k2 Cocker E2'd and MQ'd
Blue Spyder e99