The fasteners on LS tractors are metric. The table provided above is a generic torque table for metric fasteners provided in the owners manual.
If you need to determine the size of a missing bolt, you can go to the LS tractor website parts list and look up the fastener. The description tells you what size fastener you need. For example, for a step bolt on an XG3140, the specification shows; M8XP1.25X40L. That means the bolt is 8mm diameter, 1.25 thread pitch, 40 mm in length.
As far as grade of bolt, SAE grade 5 = 8.8 metric and SAE grade 8 is 10.9 metric. Metric 12.9 has no SAE grade AFAIK. If you don't know the grade of the missing fastener, find a mate on the tractor and proceed accordingly.