My boat was originally a topmasted gaff cutter, and in theory I thought that would be the best solution to the problem of how stiff/tender a boat should be. My mainmast was short and heavy, and the topmast went up another 7 metres or so, and was also pretty heavy, solid Doug fir. It was fidded so it could be "housed" by lowering it so the butt was near deck level, and secured with ropes.
On the positive side, it worked well in that it was like having 3 different boats. With the topmast up the boat was tender and handled heavy seas in light wind, a common thing on the British Columbia coast, with a slow period of roll, and I could set a lot of sail. Problem was, I didn't like getting caught in a squall with all that windage and so many sails to get down in a hurry. Housing or stowing the topmast while at sea turned out to be impractical. It wasn't too difficult to do while at anchor in a calm harbour, but I never even attempted it at sea. In the old days when life was cheap they would send a seaman aloft to pull the fid out, but I never dared, and would definitely not ask anyone else to do it.
Before I left to go cruising offshore I made the decision to leave the topmast behind, and put up with a boat a little on the stiff side. I rolled more than I would like in moderate conditions, but was grateful for the high righting moment when the weather got heavy. I never once went to bare poles in years of sailing, some of it pretty blowy. The downside was that light conditions were hard on my sails and rigging, and on me.
My new rig is going to be 3 masted, and I'm estimating that my new rig will be roughly half the weight of the old one, all 3 of them will weigh quite a bit less than the single solid fir one I had, plus the heavy standing rigging. The downside will be increased windage, or "top hamper", probably 30% to 40% more. I've come to mistrust theory and calculation when it comes to boats and rigging, buoyancy can be accurately calculated if you have the hull lines, but factors like weight and windage are always an educated guess at best. And how well an idea will work in practice is often not quite as good as it looks on paper. I expect my boat to be considerably on the stiff side but I don't plan to do any adjustment of the ballasting without some real world trials.
I feel that one of the most important considerations in how stiff or tender a boat ought to be is the point at which the boat must go to bare poles. That's a downfall of many modern boat and rigs, they're too tall for heavy weather and I know quite a few of them are taking it all down in as little as 35 knots of wind, unacceptable for offshore sailing in my opinion. I doubt there is any accurate way to calculate that, because every boat and rig is different, and the size and displacement of the boat will make a large difference as well.
Summing up, I think the right amount of stiffness is a matter of the kind of boat you have and the kind of sailing you do. Inshore and coastal sailors may prefer a more tender boat and are less likely to experience severe weather, at the expense of some speed. For people who like racing, stiffer will generally be better. For offshore sailing heavy weather is inevitable and the minimum righting moment should be enough to carry a small storm sail in storm force, without getting bowled over by the gusts. Too much can be just as bad, in the old days they were very careful to load sailing ships correctly, stowing the cargo too low could shake the rigging right out of the ship. I plan to do the rolling test and inclining test as well as sea trials in high winds, before deciding how much is right.