The expectations of the guy behind you are his responsibility, not yours. He can choose to alter his expectations. Fifty is a limit, not a target, so it’s not always possible, advisable, or desirable to acheive that speed and maintain it at all times. I wouldn’t worry about other drivers’ state of mind, that’s their responsibility, they choose to get worked up. Thirty would be unreasonable, but anything between 40 and 50 isn’t.