How to calculate the percentage yield for enantiomers in free radical halogenation?
I'll say that this is just a matter of convention, and not really worth worrying about. Your underlying method and concept is 100% correct. Your final question boils down to:


  Are enantiomers separate chemical compounds?




To elaborate, if the question was:


  Find the percentage of major product formed, counting the enantiomers separately.


Your approach would have without doubt been correct.

But since the question only says:


  Find the percentage of major product formed.


It is difficult to ascertain what the question wants. Enantiomers themselves have different chemical reactions - so they may be interpreted as different chemical products. But, your teacher has interpreted them as a single compound, presuming that optical isomers are just one compound.

Again, I guess this is just convention, because I haven't been yet able to find references to favor either definition.



Finally, to satisfy both the parties involved, and write a foolproof answer that guarantees a complete score, you can frame your results like so:


  
  Percentage of (R)-2-chloro-3-methylbutane = $x$
  Percentage of (S)-2-chloro-3-methylbutane = $x$
  Total percentage of 2-chloro-3-methylbutane = $2x$
  


where $x$ is your calculated value for a single enantiomer.