Sorry low sleep needs and yes around 3 hours and 30 minutes.
Also u might find that u will have to shorten the second nap to 20 minutes to keep a reasonable BT and after 20 minutes he will need a shorter A time too.
I know night wakings are rough we have suffered here too, Oskar has never slept through, the best he has done is sleep from BT to 3am.....hang in there.
Im going back to work soon, so this is what we started doing now to keep some sanity.
I started waking him at 7 so we start the day at the same time, first nap is 4 hours later then another 4 hours until second nap. second nap is still trial and error, but generally after 20 min nap he can do 2hours 30min and after 30-40 min nap he will do 3hours 30 mins.