Yes, cues start to get difficult at this age. My DS will sometimes yawn after an hour being up! Then he'll get a second wind. At 7.5 months you should be having no more than 3 naps, most typically two naps around 9/9:30 and 2/2:30 for about 1:15 or longer.
If I remember correctly, at 7.5 months A time for my DS was 2.25, 2.5, 2.5, 2.5 approximately.
Babies can drop the catnap once they can stay awake 3+ hours before each sleep period.
7-9 months is a difficult age to guage awake times etc. What time does DD wake in the morning? If after 6:30, i would try and keep her up until 9 for first nap. If you can give us an idea of her awake times we can be more helpful.
hugs to you. I had a very hard time from 7-9 months!