You just get yourself into trouble not using standard flange by many aspects such as custom gasket, bolt size and length, and future maintenance issues.
60" and under, use standard, regardless how small the pressure is. Also need to include equipment pressure from external loads. Check with your pipe stress engineer. Saving nothing by custom design per Appendix 2, but waiting for you is the trouble.
Over 60", custom design with "art" in mind, such as not using too large bolt diameter, watch out bolt spacing, not to wide. Limit gasket width to 1" or less. The largest flange I have designed 120".