Not an expert, but logically the shorter chain makes it easier to handle and relatively more precise.
A longer chain however, allows for a greater swing, resulting in a much heavier impact.
That however can also be achieved using a heavier ball on the flail. But that makes it heavier to lift and carry.
Using multiple balls however seem counterproductive, as it will make the flail harder to use compared to the impact you can achieve.
I suppose the idea is to make it harder to defend against, but if the defender has a shield, I think multiple balls are utterly inefficient.

So as far as I can tell there is no single optimal balance. It depends on the persons strength and agility, and I suppose it also depends on what type of enemy it should be used against. For a heavily armored opponent, a longer chain will be better to smash hard against the heavy armor, and the armor will make the opponent relatively slower to avoid attacks.

In short I think the bottom left or the one above seems best all round, if you want something more precise, other weapons will probably be preferable.
The best would probably be the bottom right, but with one of the balls and chains from the top left.

An advantage with the chain is to avoid a hard hit straining your hand and wrist. And that part can also be achieved with a short chain. But the chain also has the function that it can pass a defense that would block a cane. If you block the chain, the chain will bend and allow the ball to continue a short distance further. With a shorter chain the flail is easier to defend against.

Asking for a future self

Far as I know, flails as depicted weren’t really used in war. They may have showed up in tournament fighting, but not war. The flails that did get used in real battlefields were pole weapons. Long shaft like five feet or so, very short chain that’s essentially just a hinge, then a sort of long head something like 18" that may have had spikes or bumps.

Talhoffer wrote a manual for them, calling it a peasant flail. It has a lot of wonky binds that I would conjecture probably didn’t get a huge amount of use outside duels. In war, it was probably mostly overhead bonk attacks. Fighting against these things in a duel is a real bitch though. A skilled user can shower you in weird bullshit that’s hard to predict or handle and can’t really be done with anything else. There’s a ready stance where you hold it head-down and lean on it.

Nah just have the chain length short enough that it can’t swing back and whack your hand. That’s pretty much it.

My out of my butt possible explanation to the chain is to preserve the strength of grip because if you’ve ever hit something really hard with a solid object like a bat against another like a pole or rock, or even a bad contact with a baseball the vibrations from the impact can be painful and reduce your strength, sometimes causing you to drop the bat. Dudes might’ve had some thick gloves though to probably stop that though.

Also another butt idea, it may also preserve the condition of shaft overall because it wouldn’t be experiencing a lot of impact pressure either, only from blocking I guess and the general wear and tear of the attachment of the chain to shaft thing and the tug of swinging that bitch around.

I could also see it as an over the shield lever point where the shaft strikes the top of your opponent’s guard and the ball and chain fall down upon them.

Thanks for coming to my butt talk.

(I edited a spelling mistake. If anyone cares about edit stuff.)

Having played with my friend’s nunchucks in high school, I suggest long enough for the pointy bits NOT to reach your own forehead.

I think the chain should be the length of half the width of your opponents shield, or thereabouts. That’s just me guessing and I’m no nerd about medieval warfare.

Flails are apparently kinda shit but prolly work against shields.

I feel these are all dated and as an advanced society we have far better option. An arm length, 2cm braided steel cable will provided equap parts control and flexibility while also allowing the entire length to provide impact force.

For a handle, a hand and a half of polycarbonate tube slide over the base of the cable and epoxied in place will provide a sturdy base, you can then wrap it in sports tape or paracord and even add a wrist strap as you epoxy it.

Lastly for the head, I believe there is onlly one obvious option. This will provide both a weighty enough impact to be lethal to the man and a weighty enough insult to be lethal to the pride.

Here’s a video by historic arms manufacturer Todd Todeschini and historic martial arts teacher Matt Easton on chain flails:

https://youtu.be/Dc2wBq8JL_Y