It's not like I'm riding fast (about 45 mph) but I had to make a quick stop for a light the last couple of days and locked the rear tire both times. Not complaining, but a bit surprising as I never had it happen on my heavier bikes. The Ninja is so light that it has a lot of idiosychrocies I didn't experience before. Like, don't ride on a windy day. I do love that I can actually walk it around (I only weigh 120 lbs) and that it is super easy to control, no issues with the tire locking up as the bike stayed straight and true.

Thomas, sounds like you need to be a little lighter on the rear brake - a bit less pressure and it should help keep it from locking up on you - pick any one of the 2 or 3 threads and there are lots of opinions on the issue

I am not sure if you can adjust the pedal pressure on the rear brake but I will check and see.

The rear brake pedal is directly linked to the rear master cylinder. It is adjustable, but the adjustment sets the position of the piston at the start of the brake pedal's stroke.

The only way you could adjust the pressure it delivers to the brake relative to the pressure applied to the pedal would be to change the diameter of the master cylinder (i.e. replace it with a narrower cylinder to provide less force) so that the ratio between its piston and the caliper pistons changed.

I think it would be easier to just get the feel of the brake through practice, since the pressure level that can cause the rear tire to loose traction with the street varies with the road surface, the tire, and anything in-between (sand, water, oil, etc.)

You need to stop doing that. Much more front brake. Much less rear brake.

Thread 1 (http://www.ninjette.org/forums/showthread.php?t=23372)
Thread 2 (http://www.ninjette.org/forums/showthread.php?t=21607)

If you're locking up the rear tire, you don't just have a rear braking problem, you have a braking problem that is keeping you from stopping the bike in the shortest distance. Just using less rear brake doesn't fully fix the issue.

Generally in situations like that, I hit the front brake fairly hard, but ease onto the rear brake. It makes it easier to keep it from locking up on you. :D