I have been asked to run BOMA calculations for a large mixed use project that includes Parking, Office, Retail, and Hotel.

I have made it through the MUCA allocations and am proceeding to determine the Office portion. My question is:

I have multiple elevators and egress stairs on each office floor classified as MUCA-C (since there are retail areas on the top level of the building). When calculating the EGA for the MUCA allocation the measure line is located at the centerline of walls. However, when calculating IGA for an office use, the measure line is supposed to be at the far side of Major Vertical Penetrations. Therefore, when I run the calcs for the office floors half of the walls making up these egress stairs and elevator hoistways seems to be unaccounted for. BOMA Z65.6-2012 states that I only need to change the measure line of walls at the exterior of the building. This doesn't make sense to me. Am I doing this correctly or is there something I am missing?

Thank you!