I'm a 75 front 25 rear breaker, what should you do?
In an emergency stop situation, your front brake is going to be doing virtually all the work, the rear may actually be barely in contact with the ground, so any more than a small amount of pressure on it will cause it to lock.
It's a good idea to find somewhere out of the way to practice three stage braking:
1) Take up the pressure on the lever to light the brake light and begin to engage the front brake to feel how the bike responds.
2) Smoothly and progressively increase the pressure on the brake, continuing to feel the reaction. If it feels like the rear is starting to lift or the front may slide, release the pressure slightly, then reapply.
3) As you get to the speed you want (which may be zero), reduce the pressure, ideally releasing it completely as you reach the target speed.
The aim is to get a smooth progression in pressure giving controlled braking, rather than just grabbing a fistful and hoping, but ideally, of course, don't get into the situation in the first place. If you can't see what's ahead of the van or other vehicle in front, drop back to give yourself more space and time to react.
Also, what would be the best reaction if it was wet conditions?
Braking distances on wet surfaces are going to be longer and with a greater risk of skidding, so reduce your speed and increase the separation between yourself and the vehicle in front to get more time to react and stop.