Do you always have 40 days cycles? Then your best chance will be between 22nd August to 28 th August roughly. In a regular cycle, maximum chance of getting pregnant is around 12-14 days before your expected date of next cycle. U can calculate like that. But if you have slightly irregular cycles, this time can't be predicted. Though you can have rough idea from these: 1) LH kit test 2) your body temperature. Just the day after ovulation, your body temperature will be slightly higher 0.5 to 1 degree F. 3) your cervical mucus. It will be copious relatively, just after ovulation

If your menses are slightly irregular one cannot predict exact day of ovulation. You can use ovulation kits to know it. Consult a gynaecologist for better understanding.

If your cycle is regular  then have physical relation from day9-day18 of your cycle gives max result.. But it is of 40 day continously then  leave 14 days from behind  that means around 26 day... If is is irregular then follow a gynac advice for regularising the period