I recently was on a southbound Northern Line train which was going on
the Charing Cross branch while I wanted to go to Moorgate.
I got off at Camden Town and changed platforms. 20:20 hindsight
suggested I would have been better to wait on my original platform.

Has anybody got a suggestion on the best logic for this?

If the fare is capped, a change from Euston to Euston Square might be
sensible. The change between the two branches at Euston is not to be
attempted more than once.
--
Walter Briscoe