You seem to have the right idea.
At 6 months, mine was taking a nap every 2.5 hours or so. The "biological" best times for naps is between 8:30 and 10 for the am nap, and noon and 3 for the afternoon nap. Best target times are 9 and 1.
For the morning nap, I'd limit it to 1.5 hours or so. The afternoon nap should be 1.5-2 hours. If she needs a third nap, limit it to no more than 1 sleep cycle, or 45 mins. Of course, that's textbook. What you really need to determin is her mood after a nap. Does she wake up crying? Is she playing happily in her crib? If she's crying, chances are she hasn't slept enough. If she's playing happily and enjoying herself, she's probably had a long enough nap, even if it was only 40 mins.
Night wise, you should set a target of about 11-12 hours of sleep. Use her natural wake time in the am to set bedtime. However, if your dd is getting more sleep than that at night, and it doesn't affect naps, chances are she needs it. If she's not getting enough sleep at night, having trouble with night wakings, waking too early, etc. But she's sleeping quite a bit during the day, then you should probably wake her after 1.5-2 hours of nap time.
Hope this helps!
*HUGS*
Sonya =P