We use a fixed CI of 20 for our -300/500/700 aircraft. That gives you a descent speed on 266 in the 300/500 and 261 in the -700. We modify the descent speed to meet published arrival or ATC requested descent speeds. Some folks (certainly not I) flt 280/300 knot descents routinely.
We are supposed to be looking at getting leg specific CIs from a computer program based on a variety of factors such as descent winds etc.