Your sail manufacturer should be able to give you "rig tune" guidelines for your boat.
That includes recommended rake.
As for tension... some can give you Loos gauge numbers, some will give you a way to tell what tension you need.
But YES, under sail, mast otherwise in column and also straight throughout, if a leeward shroud is flopping in a moderate breeze, you are too loose.
MOST tuning guides will tell you how to "dock tune" for your coarse setting... but then tell you to sail a close hauled starboard tack, adjust the leeward shrouds to "x", then tack and do the same adjustment to the other side. Mostly because it's easier to add another crank on the shroud when its got less pressure on it.
Hadn't thought of the middle of the mast dropping if your leeward lowers were loose when you're on the wind, but it makes perfect sense. But then I could see in light air where that might be "advantageous" too.
And RIG tune isn't my forte' but I've recently understood why I should get better at it!