Ah, the million-dollar question. The accepted wisdom is two cups of liquid per one cup of rice. This ratio tends to be foolproof when the rice is covered and cooked by absorption. When the grain cooks uncovered in a shallow paella pan, liquid tends to evaporate more rapidly, at a rate that will vary slightly each time you cook. The exact amount of liquid also depends on the rice, how thickly it’s layered, and the diameter of the pan. For rices other than bomba, begin with the two-to-one ratio, setting aside an extra cup of liquid for each cup of rice. Before the rice goes into the oven, taste it. If it feels raw, sprinkle on about a half cup of liquid. As the rice bakes, check it a couple more times, splashing on more liquid if the rice still refuses to give. As you check for doneness, taste the bottom or middle layer of rice, not the top. And keep in mind that the rice should come out of the oven somewhat al dente, so it can finish cooking off the heat. As with the amount of liquid, cooking times may vary slightly depending on the variety of rice and the rate it absorbs liquid. Your best guides are feel, look, and taste. After a few trials and errors, you’ll get it perfect each time.