Uhhh,
If the Impulse works like most every other electropneumatic, then the LP reg is in charge of the cycling pressure, and you would use your primary or inline reg to adjust velocity. Any Impulse people around to confirm this? Adjusting the LP reg wouldn't affect your velocity, just cocking pressure, if I'm not mistaken. So the inline reg, or primary reg (if you're running just out of the reg on your tank) is what you would use to adjust your chrono velocity. Again, I'm not an impulse owner or tech or anything, but since the impy is so similar to most other open bolt electropneumatics, it would make sense.