I thought I would provide a little more information on how I choose a ratio for cheese blends. I first start with studying the constituent properties of a cheese that melts, firms up upon cooling, and re-melts exactly how I want it. Almost all the time I use part-skim low-moisture (PSLM) mozzarella cheese as a baseline. This means for whatever cheeses I blend together, I want the combined moisture content, protein content, and fat content to match PSLM mozzarella, or as closely as reasonably possible. For reference, PSLM mozzarella is 46.5% water, 26% protein, and 20% fat. I decide which cheeses to use based on flavor. If I don't like eating a particular cheese straight off the wheel or block, I don't put it on my pizza.
For experimental purposes, I collected cheese popularity data via google quite a while ago so that I had a reference for the most favored cheeses. This gives anyone who's experimenting with cheese blends a head start with what will likely turn out to be good tasting cheeses. Cheeses don't usually become popular because of how bad they are. The following list is the data I collected (rounded to a total of two digits). These values are normalized to the top ranking cheese. Think of them as baker's percentages. For instance, fontina is 10% as popular as mozzarella.
1.4 monterey jack
The cheese blend I use most of the time is 4 parts PSLM mozzarella, 2 parts muenster, and 1 part low-fat swiss; which has a combined content of 47% water, 25.6% protein, and 20.7% fat. I could sit down to a plate of any one of these cheeses and eat it straight as a meal. It was an obvious choice to use these three for my tastes.