Ahh, now it's beginning to make sense...
The one I have on hand, from The Bell Tolls for No One, it's true provenance is actually "Notes of a Dirty Old Man," L.A. Free Press, April 20, 1973. In it he writes:
You might say that if you bet $2 nine times with a fifteen percent take, that you are bucking a take of nine times 15 percent, which on that theory is bucking a 135% take, which might explain why most people leave all they brought with them at the track.
I hate math and I really just thought it was a common gambling term (it isn't, at least anymore), so I assumed it would be easily deduced. It wasn't. Until you helped.