book in the shirt sounds like a decent idea, actually. For me, I can't be bothered to get someone to hold the board for me, and I don't know that I could balance it myself, and I just do pin presses instead.
As for the specific height, what you need to look at is the purpose of board presses. From what I understand there are two. One: to target sticking points. So you want to set it to the point where you are weakest, so you can target that part of the motion. Two: to smoke your triceps/lockout. This is typically gonna be higher (3-5 board)
The exact height isn't magical, imo.