Perform a lysate titration to establish the linear range of the assay. For samples in the linear range, multiply the signal for the "phospho" spot by 2 and divide by the sum of the signal from the "phospho" and "total" spots, then multiply that value by 100 ((2 x phospho / phospho + total) * 100). Each phosphoprotein assay kit product insert contains a detailed explanation of the percent phosphorylation with specific examples.