Calling button bumps is something you practice the very first and second days of rehearsal when the cast is learning music. That's the SMs opportunity to learn the music too, and count/memorize the final notes of any song. Learn the button bumps just as you learn where the music changes are in dance numbers, because yes, there will be a light change there, too.
Now although some of this is about preparation, it's MUCH, MUCH more difficult without a conductor cam. Without a cam, the singers and conductor must do the same thing at every performance if they want that kind of cue at the end of the number to be both correct and effective. If the singer is given latitude to cut off whenever they want to, that can be a huge issue.
Talk to your conductor, and ask them to count the end of each number for you, That way it's much easier to find the "GO" even without a cam.
1-2-3-4-5-6-sev-go-eight, will give you the correct bump on eight every time, and you adjust for other time signatures.
Best of luck,
Ruth