Friends, I personally use a CI of 71 and cruise at M0.800 at any altitude.

However, the 757 max speed is M0.86 above B767's M0.84, and I've put .84 or even .85 in the 757 FMC in the last leg of flights if I have enough fuel and it always keep that speed. I think if chose to set the speed hold at M0.90 I would overspeed without a doubt. So I don't really have a problem when selecting high speeds at cruise altitudes.

What is the Cost Index you've been using fsxking08?