Typically "20% savings" would mean that you save 20% of the initial value. 20% x $600 = $120, not $100. I.e., you should multiply by (1-0.2), not divide by (1+0.2).

Edit: That has the undesirable property that common English sentences don't cancel out like you'd expect -- if you lower the price by 20% then raise it by 20% you don't end up where you started. Unfortunately, that problem doesn't go away whether "increasing/decreasing by x%" always refers to the former or the latter price; you'd require that it target's the former price in one direction and the latter price in the other, which IMO is even less intuitive than the status quo.

600 * (1 - 0.20) = 480