General aviation pilot reported a near miss with a formation flight maneuvering below Class B airspace. The pilot maneuvered away from the reported position of the formation flight.

Narrative

On Day 0 there was a formation flight of 4 aircraft that was supposed to fly over Location A. That flight departed from ZZZ at approximately XA:20. That flight was supposed to hold over Angel Island until release for the formation flyover. I departed ZZZ1 also at approximately XA:20 to meet the formation pilots at HAF. My flight was my typical flight over Martinez; Richmond; Golden Gate Bridge with established altitudes designed to keep me under the SFO Class Bravo airspace. My initial climb was to 2800 to stay below a 3000 floor. I typically descend on this flight from Point Richmond to 1800 or so to stay below the next floor of 2100. I and one of the co-pilots from one of the formation pilots studied in great detail the incident by overlaying the track of my aircraft and the lead formation plane from obtained information. We can clearly establish the 200 foot vertical separation based upon those tracks. We also listened to the radio calls.At approximately XA:24:50 the lead formation aircraft was asked to squawk their code by Norcal. At XA:25:31 I was assigned my squawk code. I was advised that I was Radar Contact at XA:25:11. At XA:29:10 I announced that I was beginning my VFR descent to 1800 that was acknowledged by Norcal; and I was requested to stay clear of the Bravo airspace. At XA:29:59 the lead formation aircraft was notified of two traffic targets. There seemed to be no response from the lead formation aircraft. At XA:30:48 I was advised of a formation of four over Angel Island that I could see on my ADS-B. I replied looking. At XA:31:31 I was notified by Norcal of a formation of four ahead at 2300 and asked if I had them in sight. I replied no visual; that I had them on ADS-B and that I was diverting to the right. At XA:31:48 I was again informed of the Flight of Four over Angel Island. Can you please go around them. I replied Yes Sir and that I was diverting to the West (which is to the right). Norcal Thanked me. Note that the flight of four was not over Angel Island at this time because this is approximately when the near miss occurred (XA:32:00) according to the flight tracks. The formation was well off Angel Island by then and they were in fact not holding over Angel Island but executing an elongated racetrack course from Angel Island to nearly Point Richmond where I had started my descent. Had I been more aware of their actual flight path and position I would have held at 2800 giving an approximate 900 foot buffer. I would also note that Norcal does not appear to give a traffic warning of my presence to the formation lead. One of the wing planes informed me they saw my target on ADS-B and the ADS-B was warning them of me. They communicated to the lead plane and believed they heard an acknowledgment that the lead had me in sight. They of course had to stay on station as a wing position. There was no evasive action taken by the formation flight. It appears that I was closest in the conflict to one of the wing position planes that had a slightly different altitude than the formation lead. The only formation aircraft with the transponder squawking was the formation lead.In analyzing the situation in such detail there does not seem to be much more that I could have done. I arrested my descent when warned of the formation flight and diverted to the right as called for in 91.113 (Right of Way rules). Had the formation flight been over Angel Island as reported by Norcal there would have been no conflict as I was well North and subsequently West of the island as I worked to fly around the reported position of the formation; however; the formation had left the island and was on their first leg of their holding track that went North bound towards me and quite far from the island where they were reported.