Dealing with distances in Ancient Rome is tough. Their measurement system isn’t the same as ours, and will anyone know what I mean if I refer to something by cubits or millaria? Likely not.
The hardest thing to this point has been reporting the distance between two ships (this in chapter 9, which is almost done). Luckily, I found that one of my old favorite web resources — www.convert-me.com — includes Ancient Roman along with all the other conversions I utilize it for. Using that got me an appropriate distance between ships in passus (the Roman “yard” or “meter”). While that’s not great, 125 passus equals a Roman stadium, which isn’t a bad imagistic approximation of the required distance (it’s just over 600 feet) in the western mind. So that’s what I’m going with for now. The two ships are a stadium apart.