For those who want to play around with the Katherines formula but don’t have the book on hand. I believe this was the final form of the equation (page 186, according to the first edition). Just plug [it into your handy graphing calculator] and chug.
Or, if you’re lazy/don’t have a graphing calculator on you, apparently java applet still exists at sparksflyup.com. However, John did mention on his new site that the applet is buggy, so… up to you!