@Clare376
There are variations in how much a listing can make, so there isn't a straightforward figure you can arrive at, or adapt a formula.
Usually professional companies charge between 15-40% for full management services, but of course there is a wide variance, say if a place takes 500 a night compared to one doing 120 a night, then remember a listing that has an average booking duration of 12 nights is likely to require less of your attention than a two nights average place would.
You would need to do your sums with a percentage based reward, work out how much hours you will likely be putting in against what the estimated take will be.
You can consider taking a set fee per booking plus look to pocket a realistic cleaning fee.
Even write in a percentage bonus if you are doing superstar business and the takings exceed a agreed target.
Perhaps to review low season versus high season operations too if there are wide contrasts.
It's always worth to review the agreement regularly to make sure things are fair once you see the finances.