Health insurance is a whole new ballgame. But let's stick to P&C, and especially homeowners and renters, whereas companies are conflicted in paying out claims, as it impacts their bottom line. When they pay you your claim, they make less profits. So they're in this conflicted situation in which they have to decide between profiting, and paying your loss. A flat 20% removes that conflict and aligns interests.
In P&C it's closer to 28%, and for most of them that means agency commissions and such. That said, I think 20% expense is a bit wishful for a startup, but I hope I'm wrong and that it works out for you.
I will just assume that your 20% doesn't include LAE unless you're writing super preferred risk.
I find that an interesting coincidence in this case...