The important mark is the advance mark. The advancer unit is relatively crudely made, and wear will increase the overall advance angle. Too much advance will cause preignition and can destroy the pistons. You can experiment with ignition advance to get more performance, the stock mark allows a safety margin for poor fuel and the like. Regardless, to set the ignition properly and repeatably for consistent performance, set the idle ("F") mark with a timing light (and best with a dwell meter as well) to get the two points plates adjusted properly 180 degrees apart and with the correct gaps (or dwell). Then rev the engine and set the fully advanced spark timing on the advance timing marks, clockwise from the "F" mark but not labelled. If you watch the marks with a timing light you'll see the advancer plate "turn" between 1500 and 3000 RPM or so, when it stops that turning you are at maximum advance and can set the timing. If your points plate fits properly you will be able to adjust the timing without changing the 180 degree relationship or the dwell of either points set.
Once the advanced timing is set, the idle timing may not be on the "F" mark. Don't depair, this is rather insignificant: the bike will still idle OK over a wide timing range and nothing will be harmed. Set the advance timing properly as the ignition timing will be at full advance during 99.99% of your actual riding (unless you choose to lug the engine).
