[Closed] chain length advice wanted

Just converted my 2015 Pinnacle Lithium to a 1x9, putting outer chainring in middle ring position on the assumption that this was best chainline. Unfortunately it's now started dropping it's chain, which I can't recall it ever doing before and certainly not on it's regular loop.

So I guess I need to remove a few chainlinks - but how many? How important is getting chain length right in terms of drivetrain wear, etc. & is there some kind of formula to calculate optimal chain length?

This upgrade uses most of the existing drivetrain, but it's first step to upgrading to extended range 1x10 with a ~44t chainring. In that case, as well calculating chain length, how will I know if I need a long cage rear mech?

Any help gratefully received.