I would demand a clause that says you will be paid a minimum of X hours per week if you've spent that time on underestimated tickets.
Eg:
even if I take on "5 hours" of tickets, but spend 40. I will at least be paid 25 hours. Basically a kind of floor on what you will be paid. Then you have discretion if you spend beyond the floor on a ticket.
Also the employer is setting themselves up for your cherry picking over estimated / fair estimate tickets and refusing to do under estimated tickets.
