I've designed a lot of copper gasketed joints, though they're all very high pressure and at cryogenic temperatures. An 8 psi plug should be relatively easy to seal using a copper washer. A few considerations:
Determine the contact stress (compressive stress) of the gasket when the bolt is torqued to a 'normal' load that some mechanic might apply. Annealed copper has a yield strength around 6000 psi, maybe a bit more depending on material. A mechanic is generally going to torque the bolt to that level regardless of what torque spec you put on something unless the assembly is carefully controlled procedurally by trained mechanics.
If the gasket compression you get using this calculation is much more than 6000 psi, the gasket will act more like a rubber gasket and will flow into all nooks and crannies. Yes, it will seal very well, but it may be very difficult to remove. If this is the case, design a gland for the seal so you know where it will flow to, or just have some other metal to metal surface prevent additional torque from forcing it to flow (ie: the metal bolt has a shoulder that ends up pressing on the threaded part and prevents additional torque from compressing the copper further).
If your gasketed joint is more like a flanged connection, you may also use the metal-to-metal technique, or you may contain the gasket in another way. Also, at such low pressure you might even consider designing it so that bolting doesn't produce in excess of a few thousand psi contact stress such that the copper doesn't flow.
