mandj, that first bleed doesn't really count when it comes to trying to calculate when your next period will be. That was a reaction to the artificial hormones leaving your system (some people have a bleed like that, others don't).
What will determine it is the date you first ovulate. Your period will then follow approx 12-16 days after that. It is almost always the date of ovulation that determines the date of your next period (I say almost as some cycles ovulation doesn't occur and you would eventually get a period anyway).
Most women vary a bit from cycle to cycle as to when they ovulate (even when not coming off hormonal contraception), so the length of their cycles will vary too.
So, if you are doing OPK tests to see when you ovulate, or if you chart your temperature/cervical fluid/cervical position and have seen the temperature shift that indicates when ovulation happened, then you will be able to predict to within a couple of days when your next period will arrive.
If you haven't been checking on when ovulation occurred then you may just have to wait and see when this first period comes, and then go from there.
If you think it hasn't occurred yet then you could start using one of these methods to see if you can spot ovulation occurring, but some people don't like to get too caught up in the charting etc right away.
I started temp charting from the day before I got my implant out and saw from the temperature rise that I ovulated 9 days later, my first period turned up 12 days after that. But everyone is different, and some people take longer to start ovulating again. That's entirely normal, just depends on how long it takes the hormones that trigger ovulation to build up in your system.
Sorry for that long post! Hope it helps.