help me! How can I get the fmax value？

When I measure the max gain of LDMOS device,I found that my network analysis can not reach the fmax frequency, so I need extend the curve,but the slop of this curve is not 20db/dec!

so my question is how can I get the fmax? still use 20db/dec to extend the curve to get the fmax?

Thank you very much! I really need your help!