All of mine are different. I'm forever switching strings, making new ones etc so the brace height on any one bow changes regularly.
I don't have a method of measuring, I just get it somewhere near a fistmele. The lower it is, the more performance you'll get however.
There's not really anything historically to go on regarding brace height, unless Ascham mentioned it and I've forgotten. No idea about Victorian times, they were weird and used arrow passes and fancy nocks and stuff. Urghhh!