Hmm. Lemme just do some napkin math
480 * 16 / 15 = 512 (high res -> 1024)
320 *16 / 15 = 341,33.. (high res -> 682) le fail
Yeah, your math is better, more general.
But really, we should not necessarily cover all resolutions, and in that sense it's smarter to just calculate it before hand and set it directly. (Not that it's especially costly though).