We are going through much of the same unfortunately, my DD is just 4 months and was at 1.45A for a long time and we are still anywhere between there and 2hrs, but basically the exact same thing happens to us...1.45 gives us 45min naps but close to 2hrs gives us 30mins. I've found I've really had to watch the clock to be able to get her A time right. I found that I had to do little increases, such as 1.50 to get anything longer without her being OT. With that said, even with that she often still cannot transition sleep cycles and still will wake at 45mins no matter what, so I just do my best to resettle and go from there.
If your DD can't go the full 3.5/4hrs between feeds (many BF babies can't) can you do a top up before nap to get her through?
We also struggle with a 12hr day if naps have not been great, so I don't force it. If she can only do 11hrs or whatever then so be it.