This HH was edited from a different thread.

Hero and Villian both start with $50 stacks, have $40 behind, blinds are $0.25/0.50, and they are alone in the hand on a 10-handed table.

Hero has position on Villian with Q, A.

Villian is probably on a flush draw.

The board is:
Q, 2, 5, 8 ($20.25)

Villian checks.

How much do we bet, and why?