Building Ground Level Floor Joists

Measure 16 inches from the center of one of the edge planks and then place the next plank across the plastic sheeting centered on the 16 inch mark.
Building ground level floor joists. Fasten blocking pieces between joists in the middle of the span to prevent twisting. The string line would be fixed in position x in the diagram below. It can also bring serious building code compliance problems and potential drainage problems due to an insufficient height of the floor above ground and lack of fall to the drains. You might consider using 4x4s as sleepers.
2x4 joists would be sufficient depending on close they are spaced no further than 12 on center maybe even a bit closer and how long the 2x4s will span from one support beam to the next. Install 14 foot long 2 x 12 inch joists. Because joists and beams will be close to or in contact with the ground pressure treated lumber used to frame grade level decks needs to have an awpa uc4a tag which means that it is rated for ground contact. Use joist hangers to attach joists to the ledger board and beam header joist with deck nails or screws.
Floor levels getting the floor level wrong can lead to on going moisture problems. Lay the planks in place along the four inch side to serve as floor joists. Space the sleepers at 24 centers. Determine the spacing of the inner joists according to local building codes and mark on the beams.
To level the entire floor it is best to start with the two end joists.