well I had the same problem as Sophie stopped showing obvious tired clues so since about 5mo I've pretty much gone by the clock- and there are many here that have had to do the same!
I looked up the average A times & nap times for her age (there's some info on this in the FAQs section at the top of the board). I took into account that she's always needed more sleep than the "average"(- she didn't drop the catnap till 8mo and the norm is around 6-7) and so worked on the premise that is the average 5mo was staying awake for say 2.5hrs, then I would aim for perhaps 2-2.25hrs with Sophie and put her down after that amount of A time. Obviously if she looked knackered after 1.75hrs then I would put her to bed, I wouldn't keep her up when she clearly needed to sleep!
It's not about watching the clock obsessively, it's a balance of knowing your lo well enough to see an eye-rub and think, hmm, well she's only been up 1hr15 and I know she shouldn't be tired yet so let's play with something else for a bit and aim to put her down closer to 2hrs. Or something like that!
hope that helps a bit more
K