STAAD Static Seismic Loads ASCE7-16, incorrect base shear

Hi, i am trying to apply a seismic load to my structure using the equivalent lateral force procedure ASCE7-16 12.8. I believe this can be done simply by adding a seismic load definition ad selecting IBC 2018 ASCE 7-16.

A response modification factor of 3.0 was selected from ASCE7-16 Table 12.2-1. The approximate fundamental period (PX & PZ) were manually calculated using ASCE7-16 eq.12.8-7, with
coefficient values from ASCE7-16 Table 12.8-2. This is a snapshot of my input: 

In the output file I found these values (highlighted in yellow) for the seismic parameter for which I also manually hand calculated exactly the same values. 

However, I found the Cs upper limit value to be larger than what I calculated as per ASCE 7-16. 

Assuming that STAAD used correct Ta period (1.29), R factor (3) and importance factor (1), the SD1 value is back calculated using ASCE7-16 eq. 12.8-3 for T<TL:

SD1= 0.066* (1.29 * 3/1) = 0.255

Coincidently, SM1 =  Fv*S1 = 0.257 

and SD1= 2/3 SM1

I theorize that STAAD incorrectly used the SM1 value instead of the SD1 value when calculating the upper Cs value. However, there is no way for me to verify this as there is not equation output for how STAAD calculates the Cs limits.  

Where can I check which equations STAAD used to calculate the seismic parameters?