Let's say that I'm play from the Black side this position:
[FEN ""] 1. e4 c6 2. d4 d5 3. exd5 cxd5 4. f4 Nf6 5. Nf3 Bg4 6. h3 Bxf3 7. Qxf3 e6 8. c3 Nc6 9. Bd3 Be7 10. Be3 a6 11. Nd2 b5 12. a3 Rb8 13. O-O
My question is: How do you know when to play Na5 and when to play a5 continuing with the minority attack in such Carlsbad structures? I'm asking in general, not just for this position. And I would be happy if you have a reference like a book where you read about the strategy of playing this.