I divided its horizontal dimension (1920 pixels) by 20.4 inches and got 93.14 pixels per inch. Should I use 93, 94 or 93.14 as the screen resolution figure, or leave it at the default 72 ppi?

Not sure I understand the question. You did the math, you got 93.14 at that output rez. So its 93ppi. I don’t understand what you mean by leave it at the default of 72 ppi.