The following plan will show the most efficient layout for the plywood. You will need 6 - 4x8 sheets.
deckplans.com/pdfs/plans/SSL12W16.pdf
The plan with plywood calls for 28 piers. This is 4 more than using 2x6 decking due to the plywood needs to be supported every 24 inches on center.
The only issue with cutting the post first is the that not every board is exactly straight. The trick to not lifting the frame out is to only install one screw at the top of the support board at each end. Then you can measure for the 4x4. When inserting, rotate the support board up, place the post and rotate the support back down.