For every expanded fairing base, which are most similar in capabilities to stock fairings, their mass shall equal the mass of the equivalent stock fairing plus the mass of the stock decoupler of one size below. (1m fairing + 0.625m decoupler, 2m fairing + 1.875m decoupler, etc)
For every non-expanded fairing base, which can't fan out like stock fairings do, their mass shall equal 90% of the mass of the expanded fairing of that size, to make them slightly more competitive.
You asked for a PR, here's my take.
For every expanded fairing base, which are most similar in capabilities to stock fairings, their mass shall equal the mass of the equivalent stock fairing plus the mass of the stock decoupler of one size below. (1m fairing + 0.625m decoupler, 2m fairing + 1.875m decoupler, etc)
For every non-expanded fairing base, which can't fan out like stock fairings do, their mass shall equal 90% of the mass of the expanded fairing of that size, to make them slightly more competitive.