Unpredictable
Even at Mrs. Hudson's rate, Sherlock was going to have to find a flatmate. And that was tricky business. He needed someone who would leave him to his work, who would not mind his hours or his scientific equipment. No students – they were too loud and very temporary.
Could he advertise?
Were flatmate searches like personals? "Consulting detective seeks flatmate. Must be willing to be run over rough-shod?
Molly would share with him. She'd be perfect if she didn't fancy him. Or like cats.
In the end, perfect was a crack shot with strong moral principles and nerves of steel.