10% is the “official” census. By the church’s own estimate, based on their attendance records and their census they range anywhere from 25% to 33% of the population
Almost no one in the world believes the Egyptian government census. It is not even considered credible by that most uncredible of international bodies, the UN
I do not know how “credible” the CIA is but they report 10% - 9% Coptic and 1% other. Where do you get your figure of 30%? I would appreciate knowing.
https://www.cia.gov/library/publications/the-world-factbook/geos/eg.html