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.