A day fills up with the naps that fit in it
The sum is simpler than it looks. Their day runs from the time they get up to the time they go down. Inside it fit naps that last as long as theirs last, separated by the stretches they stay awake.
Add that up and if you overshoot bedtime, they have a nap too many. If you fall short, not yet.
Out of that comes a number you can follow
With their waking time, their bedtime, how long they stay awake and how long their naps run, you can solve it backwards: how long would they have to stay awake for a day with one nap fewer to fit.
The difference between that number and the one they manage now is the real distance. napnap says it in minutes —«they are 30 min short»— and that can be followed week by week. «Between six and nine months» cannot.
What not to do
Drop a nap by the calendar, because a forum says seven months is when it happens. This moves when the baby stays awake longer, and forcing it early is paid for with an impossible afternoon and a worse night.
The odd days are part of the process, not the signal: an afternoon nap that will not go in, or that goes in and then pushes bedtime later. They show up weeks before the change is real.
And how you know it has happened
The spare one is usually the last of the day, and it usually goes by itself: it shrinks to a catnap, it takes some getting into, or it goes in and knocks the night out of place.
napnap counts the naps they actually take, closed day by closed day, and changes the number when it has happened — not when the calendar says it should.