Tom Van Dyke at American Creation calls attention to this fascinating letter from Dr. Benjamin Rush to Richard Price, dated 25 May 1786. Dr. Rush gives his estimate of the percentage of Trinitarians in the United States at the time:
A small pamphflet [sic] addressed by you to the Congress, and the legislature of each of the States, upon this subject, I am sure would have more weight with our rulers than an hundred publications thrown out by the citizens of this country. It will only be necessary in this pamphflet to be wholly silent upon those subjects in Christianity which now so much divide and agitate the Christian world. The wisest plan of education that could be offered would be unpopular among 99 out of an 100 of the citizens of America, if it opposed in any degree the doctrine of the Trinity. Some of the members of the reformed Episcopal Church in the middle and southern States complained of the note you published with my letter in the English newspapers. It has injured them in the opinion of some of the English clergy. You will perceive from their prayer book, that their Articles, tho' reduced in number, are equally Calvanistical with the Articles of the old English Church. [emphasis added]
Now of course there is no reason to suppose that Dr. Rush took a scientific survey of his fellow citizens; it's far more likely that he just pulled the numbers out of his ass. Nonetheless he does give us numbers in an area where guesswork has been the general rule, and his is at least a contemporary guess.
Benjamin Rush's defense of the Bible as a school book also makes interesting reading in light of contemporary controversies.