We are in the transition too, and what I do is usually allow a first A time of somewhere between 3-3.5hrs (it has to be enough so that she'll fall asleep), and then I cap the AM nap at 30-45mins, and then the long PM nap is roughly 3hrs later. So for example:
WU 6:30
Nap 10-10:45
Nap 2-3:30/4
BT 7
My DD is probably on the higher end of sleep needs, many LOs would need a shorter first nap or a longer A time in between, and possibly even a longer A to BT. This is just a rough guide, I use her cues to also help me.
I get that the first nap gets shorter and the a time following it also gets shorter but what happens once that first nap is only ten minutes?
I think with my DS once the morning nap was 20mins we just dropped it and pulled the PM nap earlier, to around noon/12:30 and then pulled BT forward also.