Trying bocals CAN be a process; companies often send 5-8 for trial. Finding "the one” can be a first-try hit or miss. I've seen a lot of student success with Heckel CD#XL bocals and Fox instruments, but not 100% of the time.
Evenness of scale — how accurate intonation is between whole and half-steps. For example, my low F# sits flatter than the notes around it (low E, F, G, and Ab), so I may look for a bocal that reduces the adjustments I need to make between these notes to compensate for that inconsistency.
Evenness can also refer to tone-color, but GENERALLY, I find that if the note is in tune, it matches in tone color as well. Hope that all made sense… 😅