I made it around 10' between the whipping. My initial piece of fabric was 128" X 60", minus hems and whipping. I just eyeballed the long sides when I pulled them out from the whipping and tried to make it look like the pics on Just Jeff's site. How long; well that's another story.
The first hem I tried to run was just plain ugly. Lots of folds in the fabric and the stitching was just yucky.
So I put it away and ignored it for a couple of weeks. I decided my problem was the long hems and my distinct lack of patience so I started doing some smaller projects like stuff sacks and the like until I got comfortable with the sewing machine. Then I ripped out my earlier hem and started over. What a difference! It took me about 30 minutes to run a hem down all 4 sides and about 3 minutes to do the whipping.
I can't say enough about doing the small projects first; makes all the difference.