Guidance from the CIEP
In response to the question of whether perfection can be expected from the proofreader, the Chartered Institute of Editing and Proofreading (CIEP) says:
That is the aim, but perfection is rarely possible. By the Law of Diminishing Returns, perfection requires inordinate amounts of time and money. It is not realistic, but nor are some clients. Even when time is tight, they still want perfection while paying only for ‘good enough’. If they did not pay for copy-editing, the proofreader can only sort out the worst problems. […]
An experienced professional proofreader, reading a copy-edited typescript, should be able to spot and deal appropriately with at least 80% of all errors but at least 90% of typos – other things being equal.
That is the aim, but perfection is rarely possible. By the Law of Diminishing Returns, perfection requires inordinate amounts of time and money. It is not realistic, but nor are some clients. Even when time is tight, they still want perfection while paying only for ‘good enough’. If they did not pay for copy-editing, the proofreader can only sort out the worst problems. […]
An experienced professional proofreader, reading a copy-edited typescript, should be able to spot and deal appropriately with at least 80% of all errors but at least 90% of typos – other things being equal.
Chartered Institute of Editing and Proofreading (CIEP), 'Standards in proofreading'