Not really.  The point of turnout is aiming at proper body carriage - a by-product of which is that the feed turn out.

> How much of this rotation should I eventually try to get out of the hip joint,

All of it.  And as much as you can all the time.

> and how much rotation should be taken up by the knee joint?

Ouch.  It hurts to read this - NONE!

> but I can't begin to get beyond 50-60 degrees of rotation from my hip.

Thats because your butt muscles are not strong enough yet.  They will get there - and as an added bonus they will get a great shape for the effort.  One thing that will help you is pulling in the stomach and pulling up under the chest.

Dance On my friend,

..tom