I do my flights using the following format.
For long haul flights, I use a lower Cost Index of 0 - 40
and for short haul flights, I use a higher Cost Index of 50 - 100
I usually have problems slowing down on my long haul flights rather the short haul. This is because, from my experience, the low Cost Index means to ascend to cruise altitude faster, and to descend faster (shorter distance from TOD) on arrival. Of course it needs to add any altitude constraints into the equation, but the reason behind all that, is to use less fuel.

I hope this helps explain what I'm talking about.

Mark