in addition to John's respond, point count method (ASTM E562) is giving u as a ferrite content in percentage and magnetic instrument is giving in term of FN (Ferrite Number).
point count method is considered kind of destructive testing and magnetic instrument test is non-destructive test. that's why normally we used point count method for welding procedure qualification and magnetic instrucment for "verifying" at plant... and sorry for I never heard about "ferrite balance" before.
Best Regards,
Noel Tan