Well I think it depends on how low on the reps you want to go.
Either figure out what you can bench for 12 reps total, or figure out your 1 rep max.
I do a 1 rep max style pryamid for strength and power like this:
8 reps at 65%, 6 reps at 70%, 4 reps at 75%, 3 reps at 80%, 2 reps at 85%, and 1 rep at 90%.
For a 12 rep max and more for bodybuilding:
12 reps at 55%, 10 reps at 60%, 8 reps at 65%, 6 reps at 70%.
So you can figure out your percentages based off either your 1 rep max, or 12 rep max.
Hope that helps.