Yep, and note that 256 units is equivalent to a quarter note (regardless of what your tempo is). Use this math to adjust.
This means picking the playback length that is close to what you want, then using the Live Start position to bump the playback forwards or backwards so that the roll ends at the appropriate place. Each of the different cymbal instruments has slightly different sampled roll lengths, so you will need to adjust rolls on different cymbals to start at different points.
Also note that, if the roll is a crescendo, you should tell Sibelius to start it at its ending dynamic. Do this by hiding the real playback dynamic after the notated dynamic. So, for example, if you want your cymbal roll to crescendo from mp to f, enter the starting dynamic as "mp~f", enter the hairpin, and enter the ending dynamic as "f". This makes the notation look correct to the player but lets Sibelius play back a roll with the correct dynamics.
Last tip: there are "loud" and "soft" roll samples. Enter text "loud" or "soft" to trigger these (or "mute" to trigger the muted release rolls). I always enter this text with a tilde in front since it doesn't need to be seen by the player. Since the two different volume roll samples play back with slightly different lengths, be sure to enter the desired text before playing with Live Start position.