What little I know about calculating swing weight (called moment of inertia by engineer types) with respect to sports equipment is that it’s pretty straightforward on something like a tennis racquet or baseball bat, but because a fly rod is affected by the weight of the line that’s being cast it isn’t as simple. For example, with the same rod/reel combo throwing a 15′ cast will have a different MOI than a 60′ cast. Changing to a heavier (or lighter) reel will also influence MOI, but not as much as adding weight to the tip of the rod (i.e. the line past the tip of the rod)
Now balance is something I consider fairly important. I like the rod/reel to balance an inch or so back from the front of the cork – right about where my thumb falls on the grip. I know that it’s not a perfect standard – again, taking 60′ of line from the reel pushes the balance towards the tip – but it’s a place to start. I find casting with a reel that’s too heavy is less tiring than when the reel is too light.
All of this is a long-winded way of saying – if it feels right, it’s right.