When to add a second brood box?

[Acebird]

I would leave it until they start building in the lid, which says they are looking for more room. Nucs don't get a hive until they put honey in the lid and the single hive doesn't get a super until there is honey in the lid.
They tell you we need more room.

Your technique may work but I would say that it is a bit late because they might also be putting burr come everywhere else and attaching top and bottom frames together.

[Oldbeavo]

If the space between your top and bottom of your frames is correct they will not build much comb. There would be only less than 1/2 an inch between frames in the hive and super. If a queen excluder is put on then the space is even less as the QE halves the gap. Even if the bees build in the lid and put some honey in it, the honey is not lost as over winter they will use this honey. We never clean out lid honey as it is a reserve if we get some decisions wrong when harvesting honey.
Bees only build burr comb where they have extra space.

[Acebird]

I don't care if they have perfect bee space or not if it is after the solstice and they run out of room they will put burr comb everywhere.  if it gets so bad that they backfill the brood nest they will swarm in the fall or late summer (never a good thing).  Bees don't have rulers and they don't read books.  There is no such thing as perfection with bees, they compromise everything.
You can learn what a bee might do but you can not predict what a bee will do.  You might predict what most will do.  And when you do you become a happy beekeeper.