IMO, the answer is simple: you cannot do it inside B16.5.
Flanges manufactured of materials other than those listed in B16.5/Table 1A cannot be marked as being in conformance with ASME B16.5.
This statement is subject of formal Interpretation 4-14.

Out of B16.5, as your personal exercise in a calculation, look at a group that has "grade 65", study that material an compare with yours. Also, you should pay attention to the temperature limitations of F65.