It all depends on how your brakes & shifters are laid out on the bars.
Start by moving your brake levers towards the stem, so that the little hook of the lever sits comfortably under your index finger. This not only gives you the maximum leverage from the single finger, it also prevents the end of the lever from overlapping your middle finger – allowing a longer pull without danger of crushing the middle finger.
To acheive this, you may need to swap the positions of the shifters and brakes. Brakes seem to come fitted on the outside, and moving them towards the stem means that you have to move the shifters too. Swapping them round avoids this problem, so I can’t understand why this configuration isn’t the standard way of doing it.
I did this on my first bike – after a couple of years with the standard configuration – and was delighted by how little braking fatigue I know get. I was so pleased, I applied the same configuration to a new build, and would never go back.