I agree.
You might be able to buy a flange made to the dimensions of a F61 class xxx ASME B 16.5 flange, but as you've discovered, you can't find a P-T table to go with it.
Try asking someone who can make it what they think??
Or use material group 2.8 which has F44,51,53 and 55 in it and state somewhere (design basis?, calculations?) that this is what you've used as it is 25% Chrome.
Compare the materials and strengths and if say F55 is slightly lower then in reality that's as good as you can be.
F61 has been around a very long time so it's a bit surprising that it's not in ASME B 16.5, but the same happens to ASTM A694 F grades which is very commonly used on pipelines to match the API 5L grades and it isn't listed in B 16.5 either.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.