Let's go back to basics: I don't know who said it but the objective of bidding is to determine "which denomination?" and "how high?". The answers to these questions may vary according to the type of game you are playing (imps v matchpoints), your partner and his/her mood, your mood, the match position and/or any number of other factors. There is no single right answer. Having said that, with a 19 point opener and a partner who can reply, it is clear you want to be in game - perhaps slam. If your partner shows spades and you have 4 of them, it is clear which denomination you want to be in - so you need to tell your partner by whatever means you have at your disposal the good news. With fewer spades, you have to make more of a judgement. Don't look for rigid rules. Look to give your partner the most appropriate information you can.