The Culture v's 40kverse

[PairO'Dice Lost]

I think the size of GCU is mentioned in the novel you just read -- the Player of Games one, as well as maximum sprint speed (something like 1200c?).

Which implies that to reach Ea from outside the Empire of Azad's space (which consists of multiple stars), the ROU must be traveling many times that. Probably closer to a 100 kilolights.

So is this a case of the author not doing the math?

No, Banks did the math, it's your recollection of 1,200 c that's off; Culture ships measure their speeds in multiple kilolights (1000 c), so 1.2 Kc is way too low. Killer-class ROUs built near the beginning of the Idiran-Culture war have a cruising speed of ~87 Kc, while it's established in Excession that modern ROUs have a cruising speed of 144 Kc and a sprint speed of 221 Kc, with standard GSVs having a cruising speed around 128 Kc. A top speed of 100 Kc for a ROU built near the end of the Idiran-Culture war like the Limiting Factor sounds about right.