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Always undercook it. If it's not done enough, it can be put back on the grill. If it's too done, you can't fix it.

Definitely agree with the "feel" method - works like a charm, and you'll soon get the hang of it with practice. Just be sure you're wearing gloves and following food safety guidelines. Along those lines, you should also be testing the temp with a thermometer - a really thin probe won't ruin the meat.

Every part of the grill will be a different temp, so it might be worth throwing some cheap burgers onto the grill and taking their temp. Record the position and temp in a log to figure out hot/cold spots.

Congrats! Welcome to the world of grilling for the masses. I've worked in a corporate situation where I might have 30+ steaks on the grill, plus another 20 burgers, and a random chicken or two. And I've worked in a fine dining establishment where I might grill that much food during an entire 4 hour dinner service. Take my advice as you will, but the best advice anyone can give you is to just jump in and start learning by doing.

1. Learn your grill. Every single one is different. If it's a flat top most places have a thermometer (looks like an air hockey paddle) to temp the grill. Use it daily to learn the hot and cool spots. memorize them and try to utilize them in an organized manner. If you're lucky enough to be using a charcoal grill (as I currently do) then you have complete control over the heat.

2. Be organized. Every grill chef I've worked with have different methods. Normally I work in sections where I know that every steak on the grill in a certain area is going to say medium rare. See if that works for you. I know that when I'd sit there as literally 40 different items came in within 5 minutes I'd take the "first in, first out" approach and just make a straight line and cook according to ticket. Both methods work for me depending on the situation. you need to find your method. But never let there be meat haphazardly thrown across the grill because the second you turn around you'll have forgotten the temps on at least half of them.

3. Let no one tell you otherwise: the thermometer is your friend. Highly trained professional chefs use a thermometer now and then just to be sure. And even if they don't, they once did. The easiest way to learn what a medium steak should feel like is to temp it, then feel it. Believe it or not you'll reach a point in a few years where you can simply look at a steak and be fairly certain of the temp.