What is your Source for William and Margaret's birth dates. William was my GGGGrandfather and by the way he was listed in 1790 King Georges Co. Tax records as William Mayhall Jr and living next to his father William.
The best dates I could calculate for them using all the Census's and Tax Records are as follows:
William b.1767/70 d.1837><44 Margaret b. 1774/5 d. 1849>