Step 1: Materials
2 shelves 30 3/8 " x 20 1/8 "
1 shelf 30 3/8 " x 12 1/4 "
2 packs of posts 43 1/4 " (4 posts)
1 pack of RILL casters
1 OBSERVATOR 39 " cross brace
Step 2: Cut the posts
First I made sure that i cut the bottoms of the posts close to the pre-drilled holes so that my bottom shelf would be as low as possible, and match up flush with the bottom of the post. The front posts will be the height of the main surface shelf, and the back posts will be the height of the top shelf. Make sure to account for the standing height of the casters, which for me was 2 1/2". I subtracted that from the finished heights of the front and back post measurements then cut them. Make sure to keep your scraps, you will need them to support the top shelf.
Next I drilled new holes in the posts for my main shelf height, since it didn't match up with the pre-drilled holes.
In hindsight I realized that you only need to drill one extra hole in each shelf, since the shelves attach with one screw per corner. But I like the look of my holes matching the existing ones.