Base Test Workout - 2000 yards/meters
4 x 100 (swim, kick, pull, swim)
4 x 100 pull
10 x 100 hard, 15 seconds rest between each one
200 easy cool down
For this workout, when you start the 10 x 100 hard set, start your watch and don't stop it until the end of the set, so you get your total time for the 10 x 100 including the 15 seconds rest. Then divide that total time by 10 and round up to the nearest 5 seconds and that is your 100 base time.
If you are using a Garmin watch (waterproof one) you can set it up to show lap time. Then at the start of the set hit the start button, then after each 100 hit the lap button to display your time, then hit again after the 15 seconds rest, or just look at your watch after the 100 to see your time then hit lap button after the additional 15 seconds rest. Then at the end after the final 15 seconds rest hit the stop bottom. Then when you download it you will have all of your lap times there to see in Garmin Connect.
you did the 10 x 100 with 15 seconds rest between each interval in 19 minutes and 30 seconds (don't forget to include the final 15 seconds rest in the time after the final interval).
19:30 divided by 10 equals 1:57, round up to nearest 5 seconds and you get 2 minutes for your Base 100 Interval time.