There are people on the GB owners forum who have installed fittings on the bottoms of their sumps to drain the oil as opposed to the suck-it-out-through- the-dipstick-tube method. So they are getting all the oil out. They have reported that 12 quarts fills the sump to the position on the stock dipstick that I described earlier--- just below the "Low" mark.
Now what some people do is drain the oil, put in the required 12 quarts, see where it comes on the dipstick, and then cut down the top of the dipstick tube enough to put the factory-scribed "Full" mark at the level of the 12 quarts that are in the engine.
I considered doing that but after talking on the phone to Bob Smith about it I elected to use the Lehman manual's method of simply putting a new mark on the dipstick at the 12 quart level. Actually I took Bob's advice and put in ten quarts, marked the dipstick, and then put in the last two and marked the dipstick again. So we have both a "Low" and a "Full" mark on our dipsticks which helps determine oil usage.