It doesn't matter. As long as the chainrings are the same size, the required chain length increases by 1" for every extra tooth.

A properly designed tandem has the length of the bottom tube (and reartop tube) chosen to let the eccentric be nearly all the way back with a new chain and nearly all the way forward with a severely worn chain. At that point, you remove a link (1" of chain) and rotate the eccentric back and keep going.

To be honest, that's what I'd thought - it seems logical. But to fill out the info in my first post, the frame came to me with a new transmission set (not "original" to the frame) with a pair of 39t transfer rings. With these 39t rings, I couldn't get the chain taut until nearly all the range available from the eccentric BB was used up. If I removed a link, the chain was too short. So I swapped out the 39t rings and fitted some 40T ones I luckily had lying around and this fixed things. Hence I started to think about calculating the the correct length and wondered if I needed more education on the matter!

Most tandems are built so that you get full adjustment from the eccentrc using rings with an even number of teeth. I had one that wasn't.

I might possibly have an example of the opposite way round!

But I wonder, does the framebuilder have a "formula", or is this just one of those "trade secrets" bits of knowledge that those in the business just acquire along the way?