Dear friend,
The laminar flame speed of hydrocarbon fuels not more than the order of 30 cm/s. This can be raised to around 5 to 8 m/s in turbulent flow.
I am not sure about this, but my understanding of explosion protection/relief is to assume the worst case, which would be a stoichiometric mixture and design for pressure relief would work on the maximium pressure increase, which, from atmospheric pressure, would give 700 kPa for a hydrocarbon fuel (more if at a higher pressure).
Vents for low pressure structures are estimated following guides by NFPA 68, aren't they ? Or are you using equations such as that by Swift and Epstein in "Performance of Low Pressure Explosion Vents" in Plant/Operations Progress Vol. 6, No.2, April 1987 ?
No person, other than a PHD in Chemical Engineering, can completely comprehend the physics involved in combustion. Thanks for the link. It was very interesting, although not very understanding for those of us with lesser intelligence.