I get the same 9.1:1 cr that Charlie gets which requires mid-grade fuel at a minimum. Ignition timing may be the main culprit in your case though. Besides the obvious of being just too much initial ignition advance, too much vacuum advance at cruise or light throttle opening would be at the top of the list and then next would be the specific curve attributes of the distributor. Unless the car is driven a lot, then premium fuel would be suggested regardless to give you a hedge against fuel degradation when the car sits.
As Tom mentions, the loss of water from the radiator cap is exactly what his engine was doing when it went into detonation mode. So the next question is “how much have the heads been milled?”. You mention 0.030” but were these virgin heads before they were last milled? Measuring the pads at the edge of the head may help to verify this. Although the compression ratio is below what I would consider the threshold, heavy milling may have pushed the actual compression ratio over that limit.