Hi
my lo managed on 3 naps at this age... she switched to even shorter am one's at 6 months with no warning - it freaked me!
anyway, what i suggest is pushing your morning nap a little later if you can (start by 15 min increments) - by this age, she should manage a 2.5 hour gap between naps. The problem with looking for cues is that they tend to change with age - and I think our lo's work out what works for them also.... so if she's showing signs of tiredness, see if you can distract her a little day by day. even an extra 15 mins to each shift should sort out your evening routine.
A good tip I picked up somewhere was that most babies are happier earlier in the day than later (although mine was definitely a morning sleeper), so if you can keep the evening "shift" shorter than say morning or afternoon, you should find it easier to cope with as well.
One final suggestion - this has been a recent discovery. my lo was suddenly waking after 45 mins every evening. so, i took a little longer settling her at night. i did the nappy change, gave a bottle and then spent about 15 mins chatting and bonding with her, before putting her to bed (this meant i worked by clock for bed time, not cues). i think some of the reason for the sudden wake ups is that around 5-6 mnths tehy work out mummy can go away (not same as separation anxiety), tehy also start getting nightmares..... if yours seems to fit this - try taking a little longer in the evenings.
but good luck, it makes a huge difference suddenly getting your evenigs back!
Jenni
Hope this helps
Jenni