Hello Dave,
I am going through a similar process trying to sort out placement for an SP7X. My depth is about 28 feet. My width is 14 feet. I have a ceiling of a little over 12 feet.
So far what I am planning is to have the rear most extent of the lifting platforms (the parts that actually engage the frame of the car and go up in the air) located about 9 feet forward of the garage door opening. I come to that distance by first determining the rear most lift point on an old 911, and then lining that up with the rear most extent of the lifting platform. From that point I then measured to end of the rear bumper of the 911, which is about 57 inches. That distance is the amount that the back of the 911 is going to hang over the rear of the lift platforms. I then wanted at least 4 feet behind the 911 as clear work space with the garage door closed. So I add to that 57 inches of overhang 48 inches of work space and get 105 inches (8 3/4'). Because I like to work with even numbers I am rounding that up to 9 feet. So the distance then between the rear of the SP7X and the garage door will be 9 feet.
With the lift in that position on the floor I have also done some figuring relating to a full size Sierra extended cab. Turn outs out I will be able to get a Sierra in and get the garage door closed, though it will be tight. However, as the space is primarily to be used for small cars up to a mid-size SUV, a tight fit for a Sierra is fine.
If this makes no sense, let me know and I will try to do a sketch, scan and post it.
Mark