I reviewed other functionally similar codes (there's a more general version which is used widely in Autoplot, but it handles more cases and is harder to follow), and found these use the correct factor of 2.
I've made the code used in checkPowerFFT.jy consistent, all are using 2 now.
We were asked to verify our fftPower routine, and after finding the script https://github.com/autoplot/dev/blob/master/demos/2023/20230209/checkPowerFFT.jy at http://www-pw.physics.uiowa.edu/~jbf/rbsp/script/chris/powerfft/checkPowerFFT.jy, I could see that it was off by a factor of 2. The code contained a comment about the factor 4 being incorrect and a factor of 2 should be used.
I reviewed other functionally similar codes (there's a more general version which is used widely in Autoplot, but it handles more cases and is harder to follow), and found these use the correct factor of 2.
I've made the code used in checkPowerFFT.jy consistent, all are using 2 now.