Chinese Gender Chart!

Ok so I am 20 years old with a 3 year old toddler boy and i think i am ready for baby number two! I am very exited to have a planned pregnancy because my first was not OBVIOUSLY (15 & Pregant) so i would love to have a baby girl ( & if i dont its OK) I want a healthy baby of course. Anyways, according to the Chinese baby chart i would have a girl if i conceive the baby in the month of October, which made me exited knowing thats next month. My question is i start my period sep 17th if i try this months fertile window and do get pregant (hope i do) does that mean i conceived in sep or should i wait until my October fertile window?