The problem with that 30% rule of thumb is that it fails to account for the number of cancelled chips available.

If there are only a few uncancelled chips known to exist of a certain chip those few may have an extremely high value. But if there are thousands of drilled chips available they are not likely to be worth 30% of the undrilled variety.

For many chips that exist in cancelled and uncancelled varieties TCR lists a value for both. I think this is a better way to handle the value of drilled chips than trying to apply a rule of thumb.