You could consider an orifice flange for, ASME B16.36.
Neverthless, Ive never seen a sound and simple approach to tacklingthis problem for B16.5 flanges, while it's done every now and then.
One way of showing compliancewith hand calcs would be to assume that the drilled hole creates an equivalent external pressure on the bolt hole, and needs to be sufficient strong acc code rules (e.g. acc. B31.3 304.1.3). However, with simple hand calcs, Im not sure if youd get all the calculations correct, as e.g. the drilled hole itself also needs to be sufficiently strong from an internal pressure point of view. FEA would then be a good resort to get all requirements in 1 calculation.