Your equivalent footing (in contact with soil) will be: B'xL', where B'=B-2eb, L'=L-2el, eb=Mb/P (Mb=moment in the B direction) and el=Ml/P (Ml=moment in the L direction). The equivalent bearing pressure will then be: (P+Wf)/B'L'. You will also need to use the B' and L' values in your bearing capacity equations. Do you get the idea? Coduto explains this in detail.