Imagine a seesaw where the balance is not even. Both sides will have advantage as well as disadvantage when we relate this to two hand casting when starting and stopping movement.
Because there is a learning curve to two hand equipment the set up is not always correct. The first thing to consider is what line is desired. The line is the product of the fly choice that must be fished in a way that is best performed to trigger a take from our target fish, with the location and conditions taken into account.
Each of the possibilities will have a different balance point on the same rod.
The reason we balance a two hand rod is based on starting and stopping forward and backward strokes. The seesaw effect. If we optimize the ease of movement then the rod will be perfectly balanced when swinging after the cast is made. We will not have to hold the angle up or down but horizontal.
There is a key phrase in casting that starts the beginning of the balancing process. It states "Short Line = Short Stroke, Long Line = Long Stroke"
A short head Skagit can be less than 20 feet long. A Traditional Spey Line can be as long as 95 feet. Since the shorter head will require a shorter stroke the key factor to stroke length will be that the distance between reel and dominant top hand will be narrower. Of course we are not going to desire a 16 foot rod for the Short Skagit as rod length is determined by line head and belly length. This evolution of lines in relation to rod length I will hold for another post. For now we need to understand that shortened rod lengths has also shortened fore grip size. The fore grip fulcrum point is the determining factor no matter what the line choice. It will be in a different location according to casting style required for your line style choice. Scandi's to Short Head Traditional's to Skagits or Overhead Shooting there is going to be a balance point that will offer a weight free start and stopping point like that seesaw that is on the center pinion with people of exacting weights at both ends.
First determine the proper line required and put only the line on the reel and put the reel on the rod. String the rod and have the typical amount of line that there will be out the tip of the rod at the moment of your forward casting stroke stop. For the short head Skagit it is going to be about 20 to 24 feet. For a Scandi it will be in the 28 foot range and for a traditional line about 40 to 50 feet. Have a kitchen food scale ready to go. Place a pivot point in the exact location that you will have your dominant hand on the fore grip. At this time the rod will tell you what is needed. If the butt is heavy you need a lighter reel. If the top is pointing down we need to add backing. Take a paper cup and start to fill with weights, pebbles or sand and place on top of the rod over the reel. When your rod is balanced on the pivot you now have an exacting total weight requirement for the reel, remaining line and backing. Now, remove the reel and remove the line from the reel. Weigh the contents of the cup and the reel and write it down. Now you can start to put on your backing. When the weight of the reel plus the backing equals the reel and the contents of the cup you are ready to install the line. If the reel is too small the line will not fit and the process will be repeated with a larger reel. If the reel is too large there will be too much space and make for a reel that spins too fast and coils our line too tight.
Most of us will opt for the trial and error method and not use a scale. It doesn't really matter so long as the end result is the same.