Hiya LL
Yes this bleed you are having now (known in 'the trade' () as a withdrawel bleed) will be used to date the pregnancy until the dating scan.
I have said before but, other than for a bit of something for you to obsess think about, knowing your due date isn't that big of a deal, because when you have the dating scan, no matter how sure you are of your own dates, MW and consultants will only take the EDD from the dating scan. Which is a PITA!
Have just been on the Wii with DS1, he is getting too good, he almost beat me!