I am looking at replacing my stock chain on a 2001 KDX 220. I am going to buy a 520 H chain out of rocky mountain, but I don't whick lenght to buy. The description reads 520/106, 520/108 etc. what does that second number mean. I counted the links on my chain and came up with 53. Do I double that number and go with a 106 or what.thanks