It's interesting. I thought DD1 wouldn't have capacity but the psychologist asked her some very simple questions:
Property and finance:
"DD1, Mum and Dad help you with your money. Do you want them to?" DD1 said yes, I'll always need help.
"Would you want them to stop helping you?" DD1 said No, I'd spend all my money on chocolate.
"Would you want someone different to help you with your money?" No, I want my Mum and Dad.
Health and Welfare:
"Do you want Mum and Dad to help you with decisions about who looks after you?" Yes
"If Mum and Dad couldn't do that would you want Nanny and Grandad to?" No, they have enough on their plate.
I might have paraphrased a bit. But it was very simple and the conclusion was that DD1 knew she needed help, wanted us to help, so has the capacity to make an LPA even though she doesn't have capacity to manage her own affairs.