Every sentence makes you place yourself. The age gap decides nearly everything.
YOU CALL THEM
chị
YOU ARE
em
At equal age, calling yourself em to a woman is common and polite.
YOU
THEM
THEIR AGE
The same age
YOUNGER
OLDER
The rule that covers the rest
When in doubt, aim older. Guessing high is almost always forgiven; guessing low is not. One exception: a woman whose age you'd be overshooting by a lot.