with the default set up I still use 1 finger for every button.
index for B, middle for A, ring for C and D, pinky for E. for C+D I slide my index finger to D and use ring finger for C, for A+B I use index + middle, for A+C I use middle+ring finger, for B+D I use index and ring(I rarely use B+D). And my pinky is always ready for boost.
Works just fine for me. :D
EDIT: I got my fingers wrong previously, so I edited it.