I would not sweat it, I think the resolution is outside of what the req_fuel is meant for anyway. Try it with 2000cc and 2100cc and see what the difference is in milliseconds.

All it will really equate to is the global scaling of the fuel map, which you'll probably need to tweak anyway. Matching injector flow to the real value is probably a much larger factor, but even that can be overcome to a degree with tuning. req_fuel just needs to get you pretty close.