There isn't a written fixed percentage. However, on my last NIC inspection i presented a comprehensive/ detailed EICR of a 3 bed house with 50% dismount of accessories and equipment to the area engineer. I was praised for doing this but was reminded that the idea of the EICR was not a fault finding exercise or a tool to make money, solely a guide on the condition of an existing installation by means of sampling with minimal disturbance to property. He instructed me that 30% was more than sufficient and if unsatisfactory conditions were found then the sampling could be increased if possible or an observation towards further investigation as long as c1 departures were made safe during the inspection.
He noted this on his inspection sheet as 'excessive information provided on EICR reporting'!!
Sounds like the NICEIC are now marking you down for being to thorough! Keep it up