I think negotiating a flat fee for managing a property is a matter of suck it and see. (And submitting an offer)
I owned a small (2 flats) property in Newcastle a few years back, The goiing rate then was 7%. I approached an agent I knew, whose rate I knew to be 5%. Very tongue in cheek I said to my lady salesperson there–“How about a 3% flat fee if I give you guys this property to manage?”
She said she’d ask her boss.
I was amazed when next day she said “the boss says you’ve got a deal”
Which goes to show that everything in business is negotiable!