I can't speak for squirrels, but our local volplaning mammals (Sugar Glider, Squirrel Glider, Yellow-bellied Glider, Greater Glider, Mahogony Glider, and Feathertail Glider, if I remember correctly) have very similar aerodynamic properties, and indeed look much like your flying squirrels - once again, we see the Bank Clerk Rule in action: i.e., if you spend 20 million years working behind a counter in a bank, you will wind up looking like a bank clerk.
They are pretty damn smart and can volplane for surprisingly long distances, but they do make mistakes - not so surprising when you remember tha they are strictly nocturnal and no-one has yet fitted them with battery-powered landing lights - and from time to time they have a disaster. For example, it's not unknown for a glider to miss its intended landing spot by a foot or so and skewer itself on a protruding stick.