Classically, even 20 hours between manastone drops was seen as quite poor. I'm not sure where they're getting the drop rate from here, maybe inferring rates from Magelo and the bilgehunter site that claims any empty evil eye was a manastone? That would put it at 5% I guess. Their choice.
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There is evidence the other way though for around 1 in 8.
https://www.project1999.com/forums/s...2&postcount=70