Just a feature suggestion, no idea if it would be possible:
multiple worker tracking. Akin to how you have your vardiff +# at the end of the address, maybe a -1,-2,-3 to keep track of mulitple workers to make sure that each one is mining properly? Of course that would require doing multiple synching/lookups on the same page, I don't know if your servers can handle that kind of load or what kind of load that would require for the active refresh to continue to work in that fashion.
That's definitely a good feature to add but as you said, we must keep the server charge as low as possible because it could make us miss blocks somehow.
Let me talk with my pool dev.
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For 6 hours we had really low chance, most of our blocks were rejected (without appearing as orphan) but we found what was causing this.
It is a bug in bitmonerod that we reported to the monero devs. This bug can affect anyone, if you want more details, PM me.
I think we found the good fix as the chance is back to really high for the last 45 minutes.