This is a great job for 2 people. Have one person measure the height for the 4"x4" post and another person cut the post. Go down the rows measuring and cutting all of the posts, then place all the posts at the same time. This may require lifting the frame up slightly in order to get the post under the support board.
If you are building by yourself, go down the rows measuring the height for the 4"x4" posts. Write the measurement on a piece of paper and on the side of the support board. Next cut all the 4"x4" posts and position them accordingly.
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