I don't have the speed books in front of me, but let's say the average speed for a given weight/altitude is .70 Mach. In a strong tailwind, you slow down to maintain the same speed and let the wind do the work, it calls for .62 Mach at tailwinds of 120 knots. Conversely, flying into a strong winter headwind, it would call for Mach .78.
Not sure who developed your CI, but ours was done by Air Canada pilots along with an actual rocket scientist. First they developed a theoretical model, using book numbers to develop target speeds/profiles. Then they had our check airmen monitor the fleet for months, showing actual fuel consumption/speed for various weights/atlitudes/ISA devitations. So that made it more realistic.
I assume you guys use ACARS for this? We have a printed book. Find your weight, flip to the altitude, check to make sure you don't exceed max ISA, then derive cruise Mach based on headwind/tailwind component. Each speed target also lists fuel consumption per mile, or some such number. Huge difference between .76 and .68. And it only adds 10 minutes or so per leg (in the 100/200, anyway).
Our 70/90 guys still fly high/fast, I suppose because of the efficiencies of flying high. Our books show FL 350 as being optimum for longer legs, but we don't often go up that high. ATC still doesn't understand why we're flying so slow, but apparently it's shaved millions off our fuel burns...