Its a numbers game in my opinion. First you have to define what is your goal. One a day? One a week? Ten a day? How many copies of book one do you need to sell to be satisfied? If your're satisfied, then leave it alone.
After that, you're going to need some hard numbers. How many people who are paying for book one go on to the rest of the series. Then how many people who pick up book one for free, move on to the rest of the series? If your goal is money, then it will be simple to calculate what is better for you. Sell 10, pick up 5 buy through? Give away 100 pick up 10 buy through? Give away 1000 and pick up 10 buy through? You won't ever know until you can get your hands on your own data. You might be able to get a general idea from some of threads and blogs where people post their promotion results. But remember, that is their book, their blurb, cover, promotion and results, not yours.
Also there is the luck factor. Selling ten might make you more money, but 100 given away puts your book in front of more people. Even if they don't get around to reading it, they still clicked on it and downloaded it. They might just hear about it later and remember that they have it.
