How to workout AEP of PMP?

Hello everyone and hope you are doing very well.

Have you guys got any idea (or any approaches to suggest) to workout AEP of PMP? Based on literature there are couple of methods using a regional estimation for the AEP of PMP (AEP as a function of catchment area, which is a curve including upper bound, lower bound and suggested value), however those approaches in large catchments leaded to a 1:1,000,000 event or even more frequent ones and accordingly it would make some problems in our "Risks" associated with extreme flood events; it actually would increase the contribution of those extreme events in our risks, significantly. Therefore I just came across to ask you guys whether or not have got any idea or recommendations how to work out AEP of PMP in large catchments, in a more realistic way. 

AEP: Annual Exceedance Probability

Many thanks in advanced.