Or even 135 if you're wary and it will get to 140 as it rests.

I'm with your instructor though - For medium, I would pull it off closer to 130 personally. (You can cook it longer but you can never uncook, obvs.) it will be a perfect medium.

His advice is very solid and jives with everything I've learned as well. I do the salt and pepper/counter rest for an hour method. If you have a good quality steak and cook it correctly, you don't need marinades. (Although, if anyone has any good marinades I would love to challenge my theory )