I came across this thread when researching EVM and decided to revive it as a distraction from studying for a few minutes.
I believe the paper you're referring to may have used confusing terminology.
I believe what is meant with "50% complete" is actually "50% funds spent" (AC/BAC).
In that case a CPI of 1,25 would make sense: if your project was underperforming by 25% (CPI 0,75) for the first half of the budget, it should overperform by 25% for the remaining half of the budget in order to stay within the BAC.
More conventionlally, if you consider "50% complete" to be "half the work done" (EV/BAC), then your project was underperforming by 25% during half of the work, which means you've spent 0,5/0,75 = 67% or 2/3 of your funds, therefore your funds remaining are 1/3 of BAC.
TCPI=(work remaining)/(remaining funds) = 0,5/0,33 = 1,5
So, in order to keep your project within initial budget you'll need to overperform by 50% for the remaining 50% of the work.