Relationship of bases in a gene to amino acids
Assuming that all 1200 bases in the gene code for mRNA, then 1200/3 = 400 is the correct calculation. 

It's important to understand that a gene will only ever be transcribed from one strand of double-stranded DNA. Think about it mechanistically - first, DNA is transcribed to RNA by a polymerase that can only move in one direction. Second, the strand complementary to a gene's transcription start site is that sequence's complement and therefore not a start site itself. Hope that clears it up. :)