How much backing to use when using braided or other expensive line is a question that is asked often. Many people have come up with clever ways to get the job done, but they sometimes don't answer the question.
Several years ago I was taught a method that uses a cheap line counter, basic math skills, information from the reel and line to come up with a very accurate answer.
It may seem a little complicated at first but after you do it once or twice it is very easy. When I worked for Gander Mountain and would spool braided line onto new reels for customers this is the method that I would use.
The first thing you need to know is what is the Total Capacity Factor (TCF) of the spool you are filling. This information is right on the box of the reel, sometimes it is even printed right on the reel itself. In this example lets use the new Curado I, as it is a very popular reel right now. The spool of the Curado I holds 155 yards of 10lb test mono. line. We need one more piece of information to determine our TCF, the diameter of 10lb test mono. Mono line are fairly consistent in diameter and you can use a common line to get this information. For this example I chose Suffix Elite Mono, and it has a diameter of .012.
To figure the TCF we simply multiply the capacity of the reel by the diameter of the line. 155 yards X .012 diameter = 1.86
So our TCF for this reel is 1.86
In this example we are going to determine how much mono backing we will require if we want to spool our reel with 55 yards of 50lb test Power Pro braid.
The next thing we need to figure is our Braided Line Capacity (BLC) it is very easy to figure this, all that is required to know is how much braided line you want to put on the spool and its diameter. For our example we are using 55 yards of 50 lb test Power Pro braided line and it has a diameter of .014, the diameter comes right off the box.
To figure BLC we just multiply the yards of braided line we want to use by the diameter of that line. 55 yards X .014 diameter = 0.77
So we now know what our Total Capacity Factor (TCF) is and our Braided Line Capacity (BLC) the only thing we have left to figure is how much backing to put on the reel so our 55 yards of braided line fill it perfectly.
The way we do this is to Subtract the BLC from the TCF or 1.86 - 0.77 = 1.09, this is the Remaining Capacity Factor (RCF). To finish the job we just need to know the diameter of the backing line that we are going to use, in this case we are going to use 20lb test Suffix Elite Mono with a diameter of .018.
We then divide the RCF by the line diameter of the mono. backing line. 1.09 / .018 = 60.5. We need 60.5 yards of 20lb Suffix Elite mono and 55 yards of 50lb test Power Pro braid to properly fill the spool.
As I originally said, it seems a little complicated at first, but if you work through it even a couple of times you will see how simple it really is.
One of you smart guys could probably whip out a spread sheet that could make this very easy.
Good Luck.