Is the reduction of caffeine in a second cup of coffee linear or other?

I use my Keurig coffee maker each morning. Most often, I drink two cups of coffee, but I use the same K-cup for both; the second cup is just a little more watery.

I assume that the in the first cup, I am getting a larger percentage of available caffeine than in subsequent cups, since the coffee hasn’t been diluted yet.

Here’s the question: When I make the second cup (or theoretically a third) is the reduction of caffeine content linear? Or does it follow some other factor? Is caffeine reduction predictable?