All brands of rice are a little different — some cook faster, some slower. Plus, people have different preferences. If the type you're using turns out a little softer or firmer than you'd like, you can try it a little differently next time. I'd say these are the bounds of decency: If you like your rice extremely chewy, you could go all the way down to 1 cup of water per cup of rice. If you like it extremely soft, try cooking for 20 minutes and doing a full natural pressure release. You could potentially go up to 1 1/2 cups of water, too.