My thinking is that I try and keep the communities I start on feddit.uk to UK specific ones or ones that have a UK angle. If it's more general, like sleep paralysis, I will put it on lemmy.world as the largest general instance (for example: !categoryiii). If it was on a specific topic I might fish round for an instance that covers the topic (no film or Star Wars instances are holding me back from that at the moment).
Sleep paralysis (lemmy.world)
!sleepparalysis