Sales development reps: pay per meeting booked or on pipeline that converts?

Payments software company in London with a team of 24 sales development reps who book first meetings for 40 account executives. The reps are on 65/35 and variable pays a fixed amount per meeting held.

Meeting volume is excellent and quality is poor. Account executives say a third of meetings are with people who have no budget or no authority. We could pay on opportunities accepted by the account executive, or on pipeline value, or on closed revenue, but each step further away means a longer wait and less control for the rep.

Where did you settle, and how do you stop account executives rejecting meetings unfairly?