Plotting frequent positions on the chart, ascertained by the most reliable methods available, is the best method of determining set and drift. It is a navigational axiom that one should do upon the chart as much as possible. Using formulas and calculations one is apt to make the dreaded math error. Plotting on the chart is much less error prone as each conclusion must "look right". Besides, doing all that math, when a simple fix will do, keeps one too long in the chart room and not on watch where one belongs.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.