percentageIn<-function(mean, sd){
  (pnorm(mean=mean, sd=sd, q=1.25) - pnorm(mean=mean, sd=sd, q=0)) /
    (pnorm(mean=mean, sd=sd, q=2.5) - pnorm(mean=mean, sd=sd, q=1.25) +
       pnorm(mean=mean, sd=sd, q=1.25) - pnorm(mean=mean, sd=sd, q=0))
}

percentageIn(1.25/2, 3)
percentageIn(3*1.25/2, 3)

percentageIn(0, 3)
percentageIn(2.5, 3)

percentageIn(-1, 3)
percentageIn(3.5, 3)