1. .66x your PRIMARY seating distance is the recommended screen WIDTH - not diagonal. So, from your front seats, that would be about a 106" diagonal for 'optimal' center of seating experience from a theater. Yet, from the rear seats, it's about a 138" diagonal for 'optimal' center of theater seating distance.
I would say, picking a number anywhere between those two is an excellent choice and allows you to pick a slightly smaller screen by sitting back or a slightly larger screen by moving forward. I think the screen size is very much a personal choice, but where you are at is a really good size IMO.