For my DF bikes i usually use two rules suggested by Eugene P Sloan who wrote a bunch of books on bikes in the the 70s and 80s. He said the saddle should be high enough to just hit your heel on the pedal while seated. When I use this method as opposed to lower my knees give me less trouble. Any higher and i find myself rolling from side to side trying to push all the way down when the pedal bottoms out. the other rule is for fore and aft placement. Get a plumb bob and determine 2 things. Find the spot on the top tube directly above the center of the bottom bracket, and then find the spot on the top tube directly below the front of the saddle. He suggests 2-3 " as ideal for this measurement. I find it is a good place to start. As far as tilt goes, I like mine just slightly lilted up in front. This prevents sliding forward and allows me to ride on the two pelvic bones as opposed to the taint. I usually have my bars a little below my saddle height, since living in a windy place, I am frequently leaning forward to minimize the drag. A question that I have is that I have seen pictures of Bromptons with the saddle as far forward as it will go, even with an extension to increase the forwardness. Why is that so? It seems a Brompton is already fairly short in the distance between the saddle and the bars nd unless an extension is used.