[coreboot] building SPD: How to decide Trfc by spec

[Zheng Bao]

DDR3-1066 DDR3-1333 DDR3-1600   DDR3-1866      unit
                                 7-7-7           9-9-9        11-11-11     13-13-13
nRFC-1 Gb                  59               74               88               103             nCK
nRFC- 2 Gb                 86              107            128               150              nCK
nRFC- 4 Gb                160             200            240               281              nCK
nRFC- 8 Gb                187             234            280               328              nCK

Hi, All,
Above text is extracted from the spec of a DDR3 chip.

I am trying to decide the Trfc. 
What does "-x Gb" mean? How do I know which row in the table I should use?
Thanks.

Zheng