There is no so much a rule as what is visually right that faucet really has little to do with where the medicine cabinet goes and for sure if it is mirrored . Show us the room and the cabinets and even the counter

There is no room to show yet. Rebuild after a fire.

Need to know to measure so wires can be installed at the right height for the light above medicine cabinet.

You need the measurement of the height of the faucet with the handle up so the door to the medicine cabinet can clear it. Add that to the height of you vanity and top, then add 1/2" - 1 " just to be safe.

That's what I would do. Have them leave the wiring for the light just hanging out from the wall - do not put in the electrical box until you know exactly what medicine cabinet your are doing and it is on site. Things can change very quickly when you assume sizes.

How are your ceilings and what height off of the floor is the on/up position of the faucet? This is the space in which you have to fit your medicine cabinet and lighting. How much space for each depends on the look of each and how much visual space you need around each to look right.

I would encourage you to consider a recessed medicine cabinet.

Do you see how awkward this is and how terrible it appears from the side????

A recessed cabinet makes more sense and if you're rebuilding, the carpenters can frame the wall to receive the cabinet.

Notice that the cabinet is a few inches above the faucet, though there is no rule about this.

I would highly recommend you get a different faucet. A faucet that is operated from the top will be constantly exposed to drips from wet hands. I put one in a bathroom in my previous house and regretted it every time I cleaned the bathroom!.

The bottom of the medicine cabinet should be high enough to clear the faucet when the handle is in the up/on position. If the cabinet is short, don't place it too low or the bathroom will look "squat". Adjust so that the shortest and tallest person can both use the mirror, but not so low that the swinging door doesn't clear that raised faucet handle.