PCA SOG Design-Large contact area

[mjs611]

I have a 13kip load on a 10"x24" block. When doing my slab on grade design, my effective contact area is way more than the 80 max they give for 50 pci subgrade. I really don't want to be so conservative to use the 80 sq in. Any thoughts on continuing the stress per 1000 lb line to the left and starting the design from there? Is there a reason the chart is maxed at 80?

I am dealing with an existing condition and would consider increasing my contact area to decrease my slab stress but that would take me even farther off the chart! Thanks!