As a quick off-the-top-of-my-head calculation, I take 300,000,000, the speed of light in meters/second (approximate), divide by cycles per second, you get wavelength in meters. In this case, parent calculates a quarter wavelength. A full wavelength is about 5.9cm. It’s easy to get off by an order magnitude in your head, but if through sheer repetition you know 28Mhz is about 10m, you’ll know you’re off if your 30Mhz calculation gives you 0.98m. And you can do the math from there to figure out that 14Mhz is about 20m, etc. Do it enough and you can mentally move some decimal points in your head to get a pretty good approximation for about any frequency. I learned it at some point fiddling with amateur radio, but here’s a page that I think explains it well:
http://www.digitalairwireless.com/wireless-blog/t-5ghz/calcu...