Thanks Kano.
"extra bit" to "standard" ratio is very close to 3:1, which is as expected.
That hash rate is only 95% of what was on screen. I'm going to lean towards one hour being too small.
Looks like I should have included the full ending output:
[2019-04-21 17:32:59.385] Started at [2019-04-21 14:15:31.643]
[2019-04-21 17:32:59.385] Pool: stratum+tcp://stratum.kano.is:3333
[2019-04-21 17:32:59.386] Runtime: 3 hrs : 17 mins : 27 secs
[2019-04-21 17:32:59.386] Average hashrate: 489341.0 Mhash/s
[2019-04-21 17:32:59.386] Solved blocks: 0
[2019-04-21 17:32:59.386] Best share difficulty: 664K
[2019-04-21 17:32:59.386] Share submissions: 661
[2019-04-21 17:32:59.386] Accepted shares: 661
[2019-04-21 17:32:59.386] Rejected shares: 0
[2019-04-21 17:32:59.387] Accepted difficulty shares: 1322000
[2019-04-21 17:32:59.387] Rejected difficulty shares: 0
[2019-04-21 17:32:59.387] Reject ratio: 0.0%
[2019-04-21 17:32:59.387] Hardware errors: 512
[2019-04-21 17:32:59.387] Utility (accepted shares / min): 3.35/min
[2019-04-21 17:32:59.387] Work Utility (diff1 shares solved / min): 6833.70/min
[2019-04-21 17:32:59.387] Stale submissions discarded due to new blocks: 0
[2019-04-21 17:32:59.388] Unable to get work from server occasions: 0
[2019-04-21 17:32:59.388] Work items generated locally: 1352476
[2019-04-21 17:32:59.388] Submitting work remotely delay occasions: 0
[2019-04-21 17:32:59.388] New blocks detected on network: 14
[2019-04-21 17:32:59.388] Summary of per device statistics:
[2019-04-21 17:32:59.388] GSI 0 (5s):520.7G (avg):489.3Gh/s | A:1322000 R:0 HW:512 WU:6833.7/m
Using the same formula, i still only end up with 479GH/s
Duration: 11847
Total difficulty of accepted shares: 1322000.0
Calculated hash rate: Diff * 2^32 / Time = 1322000 * 2^32 / 11847 = 479GH/s
I guess a better one-to-one question would be are the total accepted share reported by cgminer matching what you can see for the entire time period.
If that matches, it takes us up from 95% to 98%, but still a little off.
I'll have another one running to a unique worker for 24hr to get a better sampling of data.