I don't use such short rest periods, whats the point in inducing fatigue when you need to max speed and form?
And using some arbitary number like 8x3 isn't a good idea IMO, stop when your speed slows down, otherwise your just doing a light weight slow, which defeats the purpose of this type of training, since it's directed at the CNS rather than muscles ---> which is also where the short rest periods things comes in... ie fatigue will make you slow...
Think of speed benching as like practising a sports skill. Not something you do when your fatigued, and not something you do with 30 second rest periods IMO...
Any rep scheme can be used as long as its under 8secs in total set time. Again not so many reps that you slow down too much from fatigue.
When I do them now, I throw the bar off the hands and recatch the bar on the way down, much better for training acceleration strength and way easier on the body.
Also as far % of 1RM goes, it depends on the person. Someone who is pretty explosive will be using around 70-75%. People who are slow will be using in the 40-50% area. This is something that will change over time, so you can't always use a cookie cutter % and sets/reps