You use your LMP to determine due date. Ovulation occurs roughly two weeks before your period. The length or heaviness of your period have no bearing on conception.
All dates are estimated and can be off by up to 1-2 weeks.
Gestational Age is counted from the first day of your last menstual period thought conception doesn't actually happen until a couple of weeks give or take after that.
using a calculator it gives a range of likely dates of conception between October 26-31.