here you go, some straight facts:
https://www.dailywritingtips.com/a-historic-vs-an-historic/
including "The Google Ngram Viewer provides an interesting look at the progress of “an historic” vs “a historic.” In 1800, “a historic” barely shows. It begins its rise in the 1820s. In 1869, “a historic” is neck and neck with “an historic.” The two travel along fairly close together until the First World War when “an historic” pulls ahead and dominates until 1938. After that, “a historic” becomes the clear winner, although “an historic” and “an historical” remain in use. Here are two recent examples of the use of “an historical” in the context of educated English..."
> Style guides like The Chicago Manual of Style, The AP Stylebook, and The Penguin Writer’s Manual regard the following as correct in modern usage: “a historical event” “a historic event.”
If you don’t pronounce the h, “an” is the clear winner. Now that most people do in America, “a” is the obviously correct spelling.
e.g. attribute, increase, conduct