Hopefully, you have a design pressure and vacuum rating for your tank. These are typically in inches of water.
Next, you need to determine the vent loads. API 2000 is the standard reference for this for atmospheric tanks and walks you through it very clearly.
Once you have the design pressure/vacuum ratings and the required inflow and outflows for the tanks, you need to pick a valve and setting so that it will flow the required rate without exceeding the tank's design pressure (or inflow the required volume without exceeding the tank's vacuum rating). Shand and Jurs and Protectoseal are a couple of common ones.
If you have blanketing valves or vent valves to a vapor recovery system, you need to stagger all these set-pressures or control points within possibly a fairly small pressure band.
Unlike pressure vessels, there is no equivalent to the 10% allowable overpressure during an upset.
