I have never watched a clock, I count hours with my 18 month old. This helped us a great deal when we switched from 2 naps to 1. As an infant, he would be awake for 3 hours, then need a hap, awake for 3 hours, need a hap, and then make it to bedtime.
Around 15 months, we went to 5 hours. It is a little tough at first, but he was showing us signs that he needed that. (He wasn't falling asleep when we put him him in bed at night) Now, at 18 months, if he wakes up at 6, he has a nap between 11-12. If he wakes up at 7, he has a nap between 12-1 depending on his energy level, the time he fell asleep the night before, and other factors that play a role to help me know if it should be more like 12, or more like 1.
Also, as you said, a tired little one sleeps less. A well reseted little one sleeps more. If our little one is not sleeping well, we just keep moving up his bed time until he sleeps though the night and until 7:00. Lately, we were traveling a great deal and he kept waking up at 6. We went with this theory of moving up his bedtime from 8 to 7:30 and it worked. After about a week, he started sleeping until 7.
Isn't this sleeping bit a horibly unfair game that didn't come with any rules! Good luck!