The way I work it out is, can I gain any more by going deeper into the round?

Usually when I'm deciding if I should continue the round that boils down to is: can't win if I don't pass*>lose if round lost>first round>gain card advantage>remove carry over>cause your opponent to play out their threats whilst you hold yours>you have so much power in your hand you can beat the opponent in round 2.

Generally you should fight a lot more for round 1 as winning the first round means you govern the pace of all future rounds, if possible you want to be going into round three either with card advantage or equal cards but being the second to play as it allows you to more effectively control the opponent's threats. Unless you have a very good reason you should pass rather than go minus 2 cards compared to your opponent. Certain decks NEED to win round 1 ie, Dwarfs and you should be more willing to spend cards if you can keep them from doing so.

*Sometimes you'd lose the ability to win the game by spending a card, it's always better to risk the game than throw away your ability to win.