That requires ciphering with long division! But so do other ways unless you have a chart.
Also, it's not exactly correct, because 7 laps is not exactly 1/2 mile. If you multiply the speed/time values in the AMA rulebook speed chart, you get 1799.28, rather than 1800 (60 * PI * 2 * 7 = 2638.94, 1/2 mile = 2640). Yes, most of the time close enough is close enough, especially at the practice circle, but if I'm going to print up a chart, or set up a spreadsheet to generate one, I might as well use the correct math.
Personally, I don't even bother to think in terms of speed when not flying a contest -- I think in terms of seconds - eg - 22.0 sec/7 high, 31.0 sec. laps low.