90-90 position at trophy is a good guide - upper arm at 90deg to the body line and forearm at 90deg to the upper arm - Murray is more or less in a 90-90.
And PP - I would always recommend focus on the loading position forward rather than focus on the starting position - so starting from 90-90 is a very good way to isolate the specifics of the approach and acceleration into contact. I have taught countless players to work forward from this position with no issues around stiffness or ruining a motion.
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