You might also consider putting a cross member parallel to the shed splitting the pad into four quadrants. Smaller segments are easier to screed and protect better against cracking. You can leave the outside form parallel to the shed off and back the truck between the forms, pour, screed, back up to the second section near the shed, pour, then nail on the form that you left off and pour the other two sections. It's probably not necessary, but when I did this type of pour, I put 12" pieces of rebar through the middle of all forms that have concrete on both sides to mitigate uneven settling. Something like this. (sorry for poor drawing, it's late)