Is the margin of error in a 1st trim ultrasound based on error of measurements or assigning a due date? I hear diff things about how "off" they can be

Gestational age. It is reported as number of days that gestational age could be off, in days. Assuming that babies all mature similarly (a fairly good assumption within a week or two) this also applies to expected delivery date. You have asked at least 3 questions about this topic. I suggest you read something more definitive such as this http://www.Nlm.Nih.Gov/medlineplus/pregnancy.Html.