To do this you will need a decently sized hard back book, a wall, a measuring tape and a friend. Start by removing your shoes. Stand with your feet approximately 20cm apart with your heels and back against the wall. Place the book between your legs and raise it until it won’t go any higher. Use firm pressure but don’t go hurting yourself. Make sure the book is squared against the wall and now get your friend to measure from the top edge of the book to the floor. Do it a few times until you get consistent results. This is your PBH. If you want to calculate your saddle height from this then multiply it by 0.883. For example if your PBH is 890mm your saddle height will be 786mm (890 *0.883=785.9). There will naturally be individual differences and preferences but this is an excellent starting point.
If you don’t have any friends (poor you) then do the same as above but facing the wall making sure you are standing upright and then mark where the top edge of the book is on the wall with a pencil. Then measure from that to the floor.