>What would be the best date into pregnancy to move her?
I would have thought that this is a question you must ask her owner/breeder. I'm assuming that she is owned by someone else currently who is supervising the mating? This isn't something that you should do without guidance and support so this ought to be the best person to advise you.
I'm not a breeder and have no experience of in-whelp bitches but a few questions come to mind: do you want to wait until she is confirmed in whelp, is there a concern about reabsorption through stress, how far does she have to travel, how will she be transported, does she already know you or are you a stranger to her?
Another thing that occurs to me is whether you have a contract or agreement in place? There seems to be a high risk of something going wrong and you'd be wise to make sure that you are protected for each eventuality. Admittedly it's much less of a risk if you know her current owner but I can see a number of scenarios where this would be a lucrative scam to trap the unwary. Take every precaution and hope it's not necessary :)