How to cook grass-fed, high-fat hamburger on electric stove?
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.