Even working in the pastry industry I never had to check hydration percentages. I kinda get the meaning of it but how do you use it to determine the things you use it to determine?
There are a couple of ways of looking at hydration.
The first is in relation to the type of flour. All flours have rated absorption values established by the millers that are related to how much water the flours can handle under normal operating conditions. For example, for an all-purpose flour, it might be 60-61% (i.e., the weight of water divided by the weight of the flour equals 60-61%); for bread flour, the rated absorption value might be 62%; for high-gluten flour, it might be 63%; for an imported Italian 00 flour, it might be around 57%. For pastry and cake flours, with which you might be more familiar because of your pastry experience, the hydration values can be much lower, close to 50% or maybe even a bit lower. In actual practice, the hydration value of a given flour might be a few percent higher or lower depending on humidity, room/storage temperature, age and dryness of the flour, etc. Some people will also try to push the envelope and try to get the flour to take on an amount of water that is much higher than the rated values. They typically do this because they want to get a more open and airy crust and crumb. I have seen dough formulations with hydrations of over 90%, a good example of which is the rustic-style dough at http://hollosyt.googlepages.com/quickrusticciabattapizza
. Such a dough will be extremely difficult to handle and will ordinarily require one to use parchment paper to get the pizza into the oven (assuming they are able to actually get the dough skin onto the parchment paper). Professional pizza operators will often use less than the rated hydration values. They typically do this because the doughs are less extensible and easier to handle by their pizza makers in making skins to be used to make pizzas. Also, sometimes they use equipment like sheeters/rollers and dough presses that in many cases work better at lower hydration values.
Another way of looking at hydration is in relation to different pizza styles. For example, a New York style dough might have a hydration that is closely related to the hydration value for the flour used. A typical range will be around 60-63%, depending on the type of flour used, or a few percent lower for commercial doughs, as mentioned above. A cracker-style dough might use a hydration value in the 35-45% range, so that the dough is stiff and fairly dry and will produce a cracker type crust rather than a soft and tender one. An American style dough, which also is a soft dough, typically uses a fair amount of oil, for example, 7-8%, so the hydration value may be lower than for other soft doughs because the oil itself imparts a "wetness" to the dough. A Chicago deep-dish dough might also use a lower hydration value, for example, in the mid-40s percent range, because such doughs also commonly use large amounts of oil, or other form of fat, in some cases up to 25-30%. A Neapolitan style dough using 00 flour will typically use about 57% hydration because the 00 flour is a low protein, low gluten flour that cannot usually take on a lot more water without the dough becoming very difficult to handle. In some cases, where the Neapolitan dough is to be baked in a very high temperature oven (above 800 degrees F), hydration levels can often exceed 60% and be as high as 67%. Those levels will be too high for a Neapolitan style dough to be baked in a standard home oven.
With time and experience, you generally develop the knowledge and feel for hydration values along the lines discussed above.