You need to measure the diameter of your barrel at the gas port then get that size FSB. Usually 0.750" fits gov't and M4 profiles while 0.625" fits pencil barrels. Your numbers are off by a factor of 10.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek