Rope shrinkage from splicing

[benz]

I was making up some bowsprit shrouds from polyester-covered Vectran, and twice had my very careful measurements come up short. At last I realized that increasing the diameter of the rope by shoving rope inside of it made it shorter. I'd like to know if there's a chart or a formula available to calculate how much length is lost to a splice for a given diameter of rope. It will take out the guessing when I move on to other diameters.
Thanks
Ben