I would estimate liquids by considering what type of dish I was making. For example, creamy dishes such as skillet pastas or casseroles made with rice benefit from more liquid as some of the liquid will evaporate and form part of the sauce. Dishes that are broth-based can generally be prepared with less liquid because the goal is a lighter consistency. If I am ever in doubt, I always use a general rule of thumb which is to begin with less liquid than I believe I will need and then add additional liquid as necessary. Adjusting upward is much simpler than attempting to correct a dish that contains too much liquid.
Cooking with Rice
It is important to keep in mind when working with rice, that it absorbs liquid at an extremely fast rate. In a regular pot, the standard 2:1 ratio of liquid to rice is used; however, in a single pan meal, you will have several different components that are releasing their own liquids during the cooking process (such as vegetables, protein, and sauce).
Therefore, you can often begin with a smaller amount of liquid (approx. 1.5 cups of broth/water per each cup of raw rice), and add additional liquid as needed. You may even be able to use less liquid than the above mentioned amount if your pan contains high-moisture ingredients such as tomatoes, onions etc. The key here is to continually monitor your pan; if your rice appears dry and/or stops bubbling, simply add a small amount of additional liquid. Conversely, if your rice mixture is too watery, simply allow the mixture to continue cooking in the open position for a few minutes until some of the excess moisture has evaporated.
Cook with Pasta
Pasta differs from rice in its ability to absorb a portion of the water added to it, while relying on the remainder of the water to cook and soften. To begin, use enough water to submerge the pasta by approximately one-half inch. Typically, this will be approximately 3-4 cups of liquid per eight ounces of pasta, depending upon the type of pasta being used. Short types of pasta (e.g., penne, rotini) will typically require additional water due to the increased surface area of each piece. Longer types of pasta (e.g., spaghetti) will typically require less water, however, they do require a brief stir during the initial cooking period to prevent sticking. Similar to rice, if the pan begins to dry out prior to the pasta becoming tender, you can simply add additional water.
Choosing the Right Pan
Finally, remember the type of pan you use is important. Skillets with a larger surface area (wider) will allow liquids to evaporate more quickly than those with smaller surfaces. Conversely, pots and pans that have greater depths (deeper) will retain moisture longer. When you are cooking in a wide skillet, it may be necessary to add as much as an additional half-cup of liquid to compensate for the rapid evaporation.
However, when you are cooking in either a Dutch Oven or deep sauté pan, you should be able to follow the normal ratio of ingredients.
Ultimately, estimating liquids is not dependent upon achieving perfection; rather, it is dependent upon developing confidence. Once I become familiar with how both rice and pasta react when cooked in a pan, I will be able to prepare one-pan meals easily, creatively and without the pressure of worrying about whether or not I added the correct amount of liquid.