Don't aim too high. That's Beautiful Buzzard with 18" extensions. The Choppers Magazine that followed the build mentioned that it was too much so he shortened it to 12". Even that is really long for a stock rake frame.
Yeah, I know it's not that realistic, I had a bike with a 8 inch overstock fork in a stock frame that rode like shit, the wheel flop was terrible! I have a narrow 16 inch wheel, so I'll be fine running 8 or 10 inch tubes I hope. If not, I'll add a piece of a front fender to stiffen it up a bit