The basic required thickness can be calculated from formulas in the ASME Code, Section VIII, Div. 1. The pressure term needs to include hydrostatic pressure.
For a lot of atmospheric horizontal tanks, the design thickness will come out very small, and minimum thickness is likely to be controlled by the design of the supports, and of bending between the supports, rather than just based on pressure alone.
If the tank is subject to partial vacuum (and/or, is buried) that may control the thickness.
There are minimum thicknesses in the different UL standards as well.
Another consideration is handling and welding.