Summary or "detailed" instructions: flip frequently, and if it's still cooking too fast on the outside and too slow on the inside, adjust the temperature down a little. Maybe you'll take two or three tries to get it perfect, but such is life.
Medium-high probably means somewhere between halfway and maximum on your stove. There's no temperature, don't worry about obsessing with a thermometer. Just try something 4-5, see how it goes, and adjust if necessary. (A thermometer doesn't really help, in any case, because what you care about in the end is the power output of the stove, not the temperature the pan is at.)
Flip as frequently as you want or need to. It'll make it cook more evenly, because you're effectively heating from both sides, instead of letting one side stay cool while the other side cooks. It will also reduce the total cooking time. Notably, Harold McGee has been advocating this for a while, and it really does work - see this blog for some nice plots and cross-sections from simulations, or this Food Lab post for a nice hands-on test with burgers. You can flip as often as every 15 seconds if you want to pay that much attention to it.
If you still have trouble with the outside being too cooked (or the inside not being cooked enough), simply reduce the heat. This is universal advice, not at all specific to burgers. Alternatively, for burgers, you could make them thinner.
Yes, and Wikipedia has a brief summary of these scales (with some further details in other portions of the article and the links).
Basically, at least four of the five recognized primary "tastes" have a reference compound that other foods are compared to subjectively. For sweetness, a solution containing the test compound is diluted until sweetness can barely be detected by a human taster (similar to the Scoville scale). Sucrose is given the reference value of 1 (or sometimes 100). Sourness is similarly rated in comparison to a dilute hydrochloric acid solution, saltiness is rated in comparison to a dilute table salt (sodium chloride) solution, and bitterness is rated relative to a dilute quinine solution. (I've not heard of any similar scale for umami.)
In most cases, the reference compound is given a value of 1. The most common way to reference these scales is as the "[taste] index," as in "Citric acid has a value of 0.46 on the sourness index," which means it has to be diluted slightly less than half of how much hydrochloric acid would be for it to be on the threshold of human sourness detection.
(By the way, while these scales show us something, they are mostly useful for comparing single pure substances. For culinary purposes, their value is somewhat limited by the complex interactions among various tastes and flavor components. Even individual substances can change flavor depending on environment: for example, a complex molecule may taste relatively neutral at neutral pH, but with increased acid and thus sourness, it may acquire a salty flavor.)
Best Answer
You take a small amount out, cook it and taste it. It is the classic way, for instance, to know if sausage is going to be good after it is cooked but before you put it into casing.