I recently had a doe come in heat on September 6th and I saw my buck breed her. Then on the 11th of September she came back in heat. I didn't see my buck breed her, but I assume he did. So do I count 21 days from the 6th or the 11th?
I would start at the 6th. This way you won't be thinking she's got a few more days when she really didn't. Better to give yourself a window period rather than a specific date.
I write down every breeding date so as to keep track of when they initially came in heat, and also the short heat dates. That way I don't have to rely on my memory as to whether she had a short heat last year and was it a 5 or 7 day heat ... just like to keep a record of what everyone is doing and has done in the past.