I bought a bulk roll of Aero Plus-X. When I developed it to a reasonable contrast index, it was quite slow, like EI 50 was about right. It was also surprisingly grainy!
The Aero films are designed to developed for contrast index of 0.75 to 0.85, to make up for atmospheric haze. When you pull back to CI 0.6 you lose a good bit of speed. So in your bracketing tests, bracket towards the lower EI side.