Frederf's explanation is spot on. I had forgotten to mention that the nomograph can help you figure out your own speed/time in order to get to the right spot.
As Frederf says the nomograph gives 3 pieces of information (speed, time, distance), if you have two pieces of that info the nomo will quickly and easily give you the third.
For what it's worth; you'll get to instinctively know what "slow", "medium", "fast" means which will help you pick a speed. I always use 5 knots or less when I get a "slow" report. Having said that I always add a couple of knots to my speed 'guess' so that I have more chance of being ahead of an intended target (i.e. calculate he's moving faster than he actually is and you'll theoretically get there well ahead of him) and then I'll head back down the enemy's plotted course and hopefully intercept. It generally works well but not always as the target could change course, even slightly, and that'll throw things off, but you have to take that chance. No guts, no glory, LOL.
