This may or may not be useful to you, but I find that if my DD is quite tired she will sleep for less then if I get her to bed before she gets overtired (even if she is not getting grumpy etc..) I put her to bed at 10 - 10.30 (she wakes at 8am!!!) and if I try to streach her morning nap till 11ish I find she will only sleep for 30 mins..

She may well be lying in her cot awake from 10.30 to 11am but she never resists and she doesn't babble so I figure she is winding down and then putting herself to sleep