As far as I know, for most of the Ottoman Empire the Janissaries were the most prominent part of the Ottoman army and all of Janissary soldiers were comprised of ex-christian minorities. I'm wandering exactly what percent of Christian boys between the ages of 8-18 would be taken for the Janissaries?
The devsirme was fairly sporadic for most of its existence, targeted different areas, and varied in the number of boys taken. Basically the Ottomans tried to recruit for their expected needs rather than taking X boys per year (maintaining an army was expensive after all). So the collection was fairly rare during the 15th century, then increased in frequency throughout the 16th century. It was effectively gone by the mid-17th century, when the Janissary corps became self-sustaining. At its height it was taking place every 3--7 years.
For the percentages taken, there's a 16th century edict that says one boy aged 14--18 from every 40 households (note that this is not the same as 1 in 40 boys, as I've seen written). There are other sources that support the idea that 1 in 40 was a common number. For example, Colin Imber points to the memoirs of 15th century Janissary Konstantin Mihailovic, who refers to boys taken from Ottoman lands as chilik, which seems to be from the Persian term for "one in 40."
On the other hand, larger numbers could be enslaved during war. Mihailovic specifically distinguishes between boys taken from "enemy peoples" and the chilik taken from the Ottoman lands (Mihailovic himself became a Janissary the first way). There were a number of exceptions to the collection. For example, it was forbidden to take an only child, or an orphan, or a boy who had been to Istanbul. Exceptions could be made for Christian regions that submitted voluntarily to Ottoman rule -- Chios is one example. Other times the tribute doesn't seem to have been very well enforced. There was 26,000 households on Euboea in 1495, but only 126 boys were taken.
So its hard to say for sure the exact percentage that was taken overall. The best guide might be the Janissary corps itself, around 10,000 in the early 16th century, 13,000 in 1560, and then a big jump to maybe 30,000 at the start of the 17th century. Those numbers were maintained by recruiting prisoners as well as the devsirme. Based on those numbers and some other sources, I've seen estimates that the devsirme took between 1,000 and 7,000 boys at a time, likely from one in 40 households in specific areas, with numbers highest when the empire was at war and the Janissaries had suffered heavy casualties.