Is it not the case that Max Alt is affected by the CRZ C of G% in the FMC. The default is 5% and is the worse case. The actual MAC will tend to be very much more. Inserting the correct number will raise the Max Alt displayed and that is the real aerodynamic Max Alt. Some companies allow the actual to be inserted, others maintain the default 5% for a buffer margin.
Further, is it still the case where FAA & JAA had different 'g' margins at CRZ levels? I think it was the case where FAA allowed 1.3g buffer & JAA wanted 1.4 or 1.5g. Correct if wrong.