You calculate the maximum initial shut-in casing pressure (MISICP) based on the original mud weight before you start drilling ahead. Once you drill deeper, you may increase mud weight. With new mud weight, you are not able to use the MASICP calculated by the initial weight because higher mud weight will reduce the MASCIP. The formula below demonstrates you how to adjust the MASICP with new mud weight.
Adjusted MASICP = Leak off pressure – [Shoe TVD x (MW2 – MW1)] x 0.052