For those of you just getting to know the pace clock, here's a way to wade in. Someone like
Spokane Al, who already puts a lot of yardage in and is his own coach, can just substitute the yards in the set for some of the distance he is already swimming. Especially since his A race is IronMan CDA, he has lots of time now to monkey around with his overall plan. If you have a coach, please discuss your clock learning curve with them. Coaches are there to help, and can suggest sets to incorporate into your training plan.
When first learning to use the clock in a lane swimming situation, leave when the sweep second hand hits sixty (also known as “on the sixty” or “on the top”). Swim an easy 50, take your time. Then do the same thing on the :30 (or “at the bottom”), the :15 and the :45. In an ideal world, the times will be awfully similar. The average of those 4 times is your holding time. If you are using a multi-hand clock, begin by picking a coloured hand ("on the red", I've also heard people say "red top"), then leave on different colours.
Now try it leaving at different 5 second intervals around the clock (not necessarily consecutive 5 second intervals). Leaving on the :05, finishing on the :48 is easy subtraction to a :43. Leaving on the :35, finishing on the :19 it is easier for me to say 19 to make 35 is 16, 60-16=44. Those using a colorful clock should now be sticking with the colour they left on for the first 50.
The simplest set to swim is a set that always leaves on the top. So a relaxed set of 5 x 50 on 1:00 will have you leaving on the :60, and will take 5 minutes. And taking your time is easy. (If that happens to be too fast, it is easy to modify to going on 1:30, so you leave at the top and the bottom. Going on 1:15 will have you leaving :15, :30, :45 and :60.)
You can follow up that set by a set of 5 x 50 descending on 1:00 (or whatever interval is comfortable for you). In this case you want your first 50 to be the slowest, the final 50 to be fastest. So you must build into your set pacing and at least a 5 second difference between the first 50 and the last. The obvious extension of this is the reverse 5 x 50 ascending on 1:00. Swimming the three groups of 50's without a break is a set of 15 x 50 and 15 minutes of your workout. A nice thing about two hand clocks is that the minute hand always clicks over at the top of the clock, so you are less likely to be confused about how long your set has taken.
In a lane swim I go in knowing that I may have to forfeit my repeat time (1:00 in the example above) because of where other swimmers are. If I leave 5 seconds early, I may try to leave 5 seconds late the next time. I may just accommodate interruptions and finish the total number of repeats in the set. Or, like Monday night, I may change my repeat time entirely. I had planned 4 x 25 fly on :45. Because of the way traffic was flowing I changed them to on 1:00, and upped the intensity level a bit.
And just in case I have confused the issue for you instead of simplifying it, here's a
link to a fine article from WSU Masters Swimming.