Add more, and it takes longer, but tastes better. One tablespoon per cup thats a 1 1. It gets super rich and creamy at right about the 1. You can also halt the process early by just refrigerating it to stop the bacterial action. This is useful if you want a thinner Mexican style crema agria for drizzling over tacos or sopes. For those of you worried about cream spoiling at room temp, thats the idea its the good bacteria from the buttermilk multiplying in there that prevents the dangerous bacteria from taking over. And let the mind blowing begin. Commence countdown.