Three Greens,
You are slightly low because you need to account for vacations, days off, training and sick days. Also Check Airman are usually not able to fly full lines due to their other duties. Since senior captains get more vacation than junior F/O's and since they are older and tend to use more sick time, airlines often also require a higher captain count than F/O's. The exception being if they use F/O's as international relief pilots then they might carry more first officers.
Jet