Wish I had information to share -- the middle to late eighteenth century is the largest hole in the database at the moment.There IS one Letitia Flandreau already in the database, but Flandreau was her married name and she died was still living in the 1840's, so she's definitely not the one you're looking for.
Assuming that she's a descendant of Jacques Flandreau (which seems a pretty good bet), there are a number of possible fathers:
Jacques's son James (Junior) had several children in the 1730's and 40's.Some of their christening records are in the church records of New Rochelle.No Letitia is listed, but there's nothing to counter the possibility either.
Jacques's son Benjamin had children in the 1750's and 60's, some of whom are buried in the Flandreau cemetery in New Rochelle.The cemetery was established on his property and Benjamin was the first person buried there.If Letitia were one of his children but moved away to Pennsylvania, it makes sense she might not have been buried there.See earlier postings for more information about the state of that graveyard.
Jacques had three other sons (Peter, Elias, and Daniel) who could have been married with children by the 1740's, though we don't have any record of their families as yet.
If you'd like to send me whatever details on her that you have, I'd be happy to add her to the database in the hope that a match comes up later on...