1 Two-way slabs have been calculated for a square panel. 2  For a 2 x 1 panel, the value for a one-way panel should be used and values interpolated for intermediate proportions. 3  Flat slab design should be based on the longer span dimension. 4  For exterior panels spanning to walls, use 85%- 90% of the quoted ratio.OTES: 1  Two-way slabs have been calculated for a square panel. 2  For a 2 x 1 panel, the value for a one-way panel should be used and values interpolated for intermediate proportions. 3  Flat slab design should be based on the longer span dimension. 4  For exterior panels spanning to walls, use 85%- 90% of the quoted ratio.