That portion of pension or retirement income received by retired service personnel, residents of this State, that can be attributed to time served in the National Guard or Reserve components of the Armed Forces of the United States, is not taxable.
The non-taxable portion is determined by using a ratio of the time actually served in the National Guard or Reserve to the total time spent in military service, times total yearly pension or retirement.
S.C. Code Regs. § 117-640.3