If you read the books you will probably find equations showing ways to figure out the amount of hops to add mathematically. Hops bitterness is typically given in IBU. If you find out that beer has an IBU of 20 then that is scale of bitterness in International Bitterness Units. I won't cover the formula here as this is the simple methods to making beer. For those that loved to do story problems in math, you can go right ahead: find the formula on the web and start computing your hops amounts. I still just use 2 to 2 1/2 pinches to give me the level of bitterness I want. Bitterness hops contrasts with the sweetness of beer and is added at the start of the boil. Flavor hops is generally added at 15 to 20 minutes before the end of the boil. Aroma hops, for smell, can go in at the last minute of your boil.
Since I'm seeking to reproduce (clone) several German beers I can narrow down my list of hops by going with German hops only. Scheer tended to go with only Perle and Cluster for the 2 beers I loved. Also, Paulaner shows the hops they use for their particular beers. I could just try to clone the recipe by going with their selection of hops.
Another example is the German Pilsner beer. If I had to do a Pilsner beer I'd choose Perle hops for bittering and a Saaz hop for flavoring and aroma. A German Pilsner has a high hop bitterness with low diacetyl. It has very low malty flavor.