Start with the specific effect rather than a universal formula
The official material uses Proc Chance in several item, rune and ability contexts. Those mentions establish that particular changes happened. They do not establish that every effect uses the same base chance, stacking rule or trigger eligibility. A complete general formula remains 待确认 in the supplied evidence.
Identify the effect named in the tooltip before comparing percentages from different pages. An item's own proc chance is not automatically the same quantity as a character bonus to proc chance. Likewise, an ability-specific hotfix is not a universal conversion rule. Keeping those labels separate avoids a false calculation made from individually real numbers.
Read Umbrothuun as a patch sequence
Version 0.20.6 adds a 100% Proc Chance stat to Umbrothuun, Hood of the Dead. The later 0.21 announcement reduces the item's proc-chance stat from 100% to 20%. The newer change supersedes the older value for the verified patch context. Quoting both values without their versions would make the equipment appear contradictory.
The 0.21 note also adds missing all-resistance information for the same hood. That is a separate item adjustment, not proof of a new proc-trigger condition. The collected passages do not reproduce the entire final item tooltip. Its full set of current interactions therefore should not be reconstructed from these two changes alone.
Use the Fan of Knives hotfix narrowly
The 0.19.6 hotfix describes a Life Lost proc that could start at 100% and decrease when affected by Additional Proc Chance. It fixes that behavior and reduces the listed Life Lost proc chance from 15% to 5%. The developer frames the latter as a rebalance so the effect works as intended. These details explain why an old demonstration of that effect may not be representative.
This does not prove the eligibility of every target or the scaling formula for every other proc. The earlier 0.19.5 announcement describes the added effect in terms of non-elite targets. Preserve the ability and target context when citing the numbers. Do not turn a Fan of Knives correction into a promise of instant boss kills.
Separate rune availability from a complete stacking model
The 0.21 update adds Kyn under Abyss runes and labels it Increased Proc Chance. That confirms the named rune and the broad effect category. The source does not supply a worked example of every interaction with items, abilities and global bonuses. Exact additive or multiplicative behavior remains 待确认 unless supported by a particular effect's evidence.
Older armor notes also mention proc-related bonuses, including an Iliossia Set change in 0.19.4. A historical set value is evidence of that update, not a complete current build recommendation. Inspect the current tooltip before using an old item table. No unverified cap or universal best target is supplied here.
Distinguish Proc Chance from Rapid Attack
Instant Attack Chance becomes Rapid Attack Chance in 0.21 and has its own primary-versus-other-ability rule. That rename does not make it another name for Proc Chance. The official patch lists these as different changes. A build that mentions both needs both dependencies checked.
For a practical comparison, record the named effect, target context, equipment source and patch before adjusting a chance stat. Compare the same effect rather than mixing different trigger descriptions. This is a method for collecting evidence, not a measured probability model. The broader build guide covers the related item and ability changes.