This is why it always pays to take lots of photos, and review them later. Here is a photo I took at Miantonomi's Cave on summer solstice at 7:26 PM. The cave was dark at 7:37, and sunset was at 8:22. I was expecting to see some solstice indicator closer to sunset. Evidently, the final light pattern occurs about 55 minutes before sunset on summer solstice.
This may be due to the structure of the site, since the windows are behind a boulder on a wide platform. As the sun sets, the edges of the platform and boulder cut off light to the windows. This photo was taken at 7:20 PM on summer solstice.