Specific rotation is a physical property like boiling point and can be looked up in references.
The most important factor is that two enantiomers will have the same magnitude specific rotations but in opposite directions. This type of mixture is called a racemate or a racemic mixture . The specific rotation of a racemic mixture is zero. Well the fact that the sign is negative tells us that in this case the R enantiomer is the dominant one. Try it out for the above example.