N. SUBRAMANIAM BOOK OF RCC DESIGN GAVE THIS SYNTEX
*SCALE FACTOR = (0.16/2) × (1.5 / 5) = 0.024SPECTRUM CQC 1893 X 2.078 ACC SCALE 0.024 DAMP 0.05 MISSOIL TYPE 2
IN THIS CASE ACC SCALE VALUE IS CALCULATED USING Z/2*I/R BUT IN STAAD HELP CONTENT VALUE OF SCALE FACTOR IS DEFINED AS "Linear scale factor by which the spectra data will be multiplied. Usually used to factor g’s to length/sec2 units. This input is the appropriate value of acceleration due to gravity in the current unit system (thus, 9.81 m/s2 or 32.2 ft/s2)".
WHICH ONE IS CORRECT?
Base shear value shall be directly proportional to both the factors so It doesn't affect the result where you assign 9.81 and where Z/2*I/R. However I would recommend you to follow STAAD help as they are the one who developed the software.