Are you asking about balance for casting or balance for fishing?
There are two purposes of a fly rod and reel. The first is to cast and the second is to fish.
Whenever the question of rod and reel "balance" is raised, there are those who will maintain that the reel balances the rod during the cast. This is a carry over from spin and casting rods and reels where the line weighs virtually nothing AND the cast is always made when the line has been wound all the way in. So in spin and casting rods a reels the amount of line and the weight of the combined line and reel is fixed. Regardless of the length of the cast, the rod and reel has a fixed balance point.
This is not so with fly casting. The amount of line varies during the cast and the length of line that is cast will vary from cast to cast. In addition, when we pick up line and recast , we do not start with all the line back on the reel. Indeed, it is advantageous to begin with some line out of the rod tip. So the balance point is not fixed but varies throughout the cast.
Secondly, there is the mistaken belief that a "balanced" fly casting outfit will somehow allow us to cast further and more effectively. This is not true. In reality, a rod without a reel with the line lying loosely on the ground is the most efficient way to cast. The reason is that the reel and line held on the reel contribute nothing to the cast. They actually hinder the cast by adding mass to the casting system that has to be accelerated and decelerated during the cast.
If the cast were made like a teeter totter with a pivoting of rod at the balance point, I can see the need for the counter weight of the reel. But that is not how a cast is made. The power stroke of the cast is made with the wrist locked and not with the rod pivoting like a windshield wiper.
Charles Ritz wrote in A Fly Fisher's Life (1959):
Fly Angler's OnLine "Bamboo Rods Part 29"
"Before describing my methods of examining rods, here are a few considerations I believe to be of importance.
Let us first rid ourselves of a widespread idea, which I have often had occassion to point out as false or, at least, much exaggerated: the reel does not balance the rod; though in the past when rods were ten feet or more, very long and heavy, a reel as a counterweight did produce the illusion of balancing the rod in the hand; but it is the line which plays the principal role owing to its weight and the shape of its taper. It is, indeed, on the line that the rod depends above all for giving its maximum, and yet retaining its balance. The ideal would be to be able to fish with the reel in your pocket."
As Ritz noted, if you remove the reel and place it on the ground and cast just with the rod and line, it is much easier to cast. I have done that and I agree with Ritz, the reel is not needed to balance the rod for casting. The feel of the cast is actually the line causing the rod to bend. It is the moving line outside the rod that "balances" or provides the dynamic resistance that causes the rod to bend.
I realize that there will be those that will not believe Charles Ritz or what I have written. You owe it to yourself to try casting with the reel on the ground as I did.
Where balance is important is after the cast in fishing. Balance is most important in nymphing in the form of "high sticking" where the rod is held up and the rod tip is elevated at an upward angle. Plus we have the rod and reel at the end of our arm which is also extended. So the effect of rod balance is exaggerated. I want a rod that is tip light so that it's natural attitude is tip up in that situation. When the rod naturally assume that attitude, I don't need to force it in that position. A tip heavy rod is very tiring especially if it is a long rod of 10 ft. That is why I use a very light long rod when nymphing large rivers. For me it is a 5 wt. 10 ft. GLoomis GLX Classic that I use to fish the Madison River in Montana.
For longer casts, with more line out, the reel gets lighter and the balance of the rod held statically in the hand becomes more neutral and the rod tip is then easier to hold in a lower position to follow the drift.
The longer the rod, the more important "balance" is because of the lever effect.
There is also swing weight that is separate from balance. A rod and reel that are "balanced" but are heavier has a higher swing weight; a larger mass begets a greater momentum.
So I want a light rod with a light reel to lower swing weight but still be tip light when high sticking.