Generally, they display similar pregnancy patterns in comparison to previous litters. But there are so many factors that could effect it--size of the litter, environmental stress, etc...
Being as you have no idea when she really tied other than the one time, I'd say that it's going to be pretty hard to time her projected whelp date with complete accuracy. One of
many reasons why you supervise ties and keep them to a minimum, which you failed to do. Allowing them to tie unsupervised, many times is definately not advised.
they tied the first time on 29th of july (that i know of..i only have one male and they are always together) and they tied many many times in the next 3-4 days..