I calculated my foc on goldtip.com. I put in 5" duravanes as my fletching. I shoot the 4" bohning vanes. I put in the 5" duravanes in because they weigh 34.5 grains all together and my bohning 4" vanes are 36 grains. Should I have put in the 4" duravanes since my vanes are 4", or should I just get the vanes closest to the vane weight?

With the 4" vanes my foc is 11.7. With the 5" vanes my foc is 10.53.

Would the foc be calculated by the weight or is the the length of the vanes? Thanks