The point was to measure the proportions of the unchurched. So, yes, I did lump agnostics from Protestant backgrounds into "unchurched from Protestant backgrounds," just as Agnostics from Catholic backgrounds are lumped into "unchurched from Protestant backgrounds."
(I did use very round numbers. For instance, I only left 3% as neither from Protestant or Catholic backgrounds. Affiliated Orthodox Christians, Muslims, Jews, Buddhists, Hindus, and Pagans comprise about 5% of the population, even counting neo-pagans, and post-Christian practitioners of eastern religions and others who fit the description of unchurched people from Protestant or Catholic backgrounds... The number of disaffiliated Orthodox Christians is probably so small, it's insignificant whether I have lumped them in with disaffiliated Protestants or disaffiliated Catholics, but I suppose I've lumped them in with someone.)
Jews: 4 million
Muslims: 1.6 million (Some sources claim as high as 6 million, but these apparently exaggerate, and may consider all people from Islamic lands, including Christians from Lebanon, Syria, Egypt and Iraq as Muslim.)
Orthodox Christian: 4 million
Hindus: 1 million
Unitarian: 1 million
Atheist: 1 million
Hindu: 1 million
Other: 1 million
Total from non-Christian or Orthodox Christian backgrounds: 15 million, or 5%