Why does the calculation of how often Player Y should bluff a Queen to make Player X indifferent to calling with a King not start by including the overlay in the pot?

The pot size is 2 units, and the bet size is 1 unit, so I would think that the equation would be:

(b)(3) + (1-b)*(-1) = 0 ==> b = 1/4  where b is the frequency that Player Y is bluffing

I understand that this is not how many Q's that Player Y will be bluffing, but the overall bluffing frequency, given Player Y will always bet an A.  However, I don't know how to convert that number into a frequency of Q's that Player Y will bluff.  I know it's very basic, but for some reason I'm having trouble. 

In MOP, they do some kind of direct calculation involving ex-showdown value that I don't quite understand (yet).