According to Taking Charge of Your Fertility by Toni Weschler you should use the following formula:
If you are charting and know your ovulation date (in your case you know it within a couple days), simply add 9 months to that day and then subtract 7 days from that date. If you ovulate around day 14 then your due date calculation will be the same as the LMP calculation. Since your cycle is a bit longer, it will be different from the LMP calculation (the one that you get from the online calculators) by a few days.
Hope this helps!
My LMP was 12/19 and ovulation was around Dec 31, so my due date is Dec 24ish.