I'll help since you have no answers from any "experts" yet. Brooks sells belts from different makers. And you really should try them on because they aren't as consistent as expectations would have you believe. Last time I bought a belt was at BB, I tried one on, and wanted it to be slightly smaller, but the actual length was fine. So I grabbed 3 or 4 more belts, laid them all out side by side, and found one whose holes were all offset by a half inch or so compared to the first one I tried, tried that one on, and the fit was spot on.
For a 30 or 31" waist, and if forced to order online, I'd go with a 32" belt.