The twist being that at the end of the month, I look at my Timesheet entries, sum up all the project hours, and then I round them up to the nearest half-day (a half-day being 4.5 hours). 

When I am creating the invoice monthly, I am creating it manually (not generated through the project) so that I can edit the amount accordingly.

Only problem is, there seem to be no way of manually flagging timesheet entries as Billed!!

Help appreciated.