Find a manufacturer of an emergency vent that fits your size manhole. They should have flow curves of pressure vs flow for each manhole size. You might try Shand and Jurs, Protectoseal, Clay and Bailey, Varec, etc.

The API-650 provisions for working pressure, design pressure, overpressure for venting, and combinations with wind, etc., have all been revised through the years.
The API-2000 venting requirements have been updated as well.
So if you apply the current codes to an existing tank, it may or may not calculate out.
I would think those fire-exposure heat-input numbers are pretty approximate at best, so I wouldn't be in too big of a hurry to update it just based on code updates.
If the existing manway is a shop-fabricated long-bolt manway, there is, or was, an equation in API-2000 for flow rate. It may have been omitted or moved to an appendix.
If you're adding an emergency vent to an existing manway, then see the Shand & Jurs, etc., information.

I realize this is an older thread but thought an little information may be appreciated. UL specification 142 for Aboveground Fuel Storage Tanks no longer recognizes long bolt manways as an acceptable form of Emergency Venting for newly manufactured tanks. An appropriate fitting with Emergency Vent is allowed to be installed in the top manway lid, but not the manway lid as the emergency vent.

I'm Darren Sanderson, QC Manager with Hall Tank Company in North Little Rock, Arkansas, USA and I am a UL Standards Technical Panel (STP) Member.

Above suggestions are good showing the Emergency Vent suppliers that may offer great references. I'll add the company Morrison Brothers too. Both Clay & Bailey and Morrison Brothers have representatives who also are UL STP members.