Hi,
As the "Can Modify Data" settings cannot be configured at cell level (only by CV, row or column definition) and your requirement is data dependent, I don't think there is an easy way to achieve that.
Also, a solution like that could potentially create a situation where a user has previously input a comment to explain a Var% > 10% and due to data changes the Var% is no longer > 10% meaning that the system would have potentially kept an outdated comment that the user would not be able to update/delete because the cell would be "locked" (i.e.: would not allow updating as the Var% was no longer >10%).
Having said that, if you have a process in place to overcome the above scenario and you believe the requirement is a must and justifies the complexity involved and assuming you do not need to use member expansion in rows (e.g.: A#AccX.Base) you could try placing an XFBR on the Member Filter of the Row Overrides section of the commentary column.
The XFBR would receive the POV and the threshold as parameters, calc the variance for the given POV and return:
An empty string if the calculated variance was over the threshold (i.e.: no change to the original POV) or something like "GetDataCell(U8#|CVUD8|)" so it returns the same POV but using a GetDataCell which would in principle turn that cell read only.
I've not tested this solution but from a technical POV might be worth to give it a try.
Regards,
Fred