The OC account does not "fall off" based on any credit report exclusion period.
Each adverse item reported by the OC becomes excluded from your CR at its own individual date.
If the OC never reported a charge-off as an adverse item, they had no requirement to have ever reported a DOFD.
Does the OC account report a charge-off? Any OC charge-off and collection on the same debt would have the same CR exclusion date.
Once the debt collector reported their collection, they were required to have reported the DOFD on the OC account within 90 days of that reporting.
With no OC reporting, they were required to have contacted the OC and attempt to obtain the DOFD.
The CRA can only estimate exclusion based on the DOFD reported to them, so any issue of CR exclusion of the collection requires knowlege of the DOFD of record in your file. To dispute the accuracy of credit report exclusion of the collection, you would need to know the reported DOFD.
Estimated dates are produced by the CRAs, and can be any time prior to the max statutory date, so cant be used to fix a definitive date.
