I work as an engine performance development engineer for a major automotive OEM. Even with all of our fancy equipment it is still good to know the old rule of thumb.
In general I use '1/2 pound of fuel per horsepower per hour' or (0.5 lbs fuel)/(1 hp * 1 hour).
Using that you'll get: 210 hp * (0.5 lbs fuel)/(1 hp * 1 hour) = 105 lbs fuel per hour. The density of gasoline is approximately 6 lbs per gallon, so you're left with an approximation of 17.5 gallons per hour. Given the antique design of these engines it isn't surprising that your value would come in at least 10% over this rule of thumb since the efficiency is so much worse than modern engines. Additionally, you're probably operating rich which would shift the guess.
The point of all of this rambeling is that you can get a quick mental conversion between horsepower and fuel flow with one simple number. If you divide your horsepower by 12, you'll have your approximate GPH fuel flow. If you multiply your fuel flow (in GPH) by 12, you'll have your approximate horsepower output. It's not exact science, but it's something you can quickly do in order to sanity-check against your instruments. Obviously, don't be dumb try to cut your fuel minimums too close using approximations