Recommend rather than having crew size increase step-wise at each new rank, allow incremental increases in crew size as the player gains experience while progressing within a rank, maybe at a linear rate between each rank that increases as XP increases. This would benefit in a number of ways:
First, it would allow players to effectively sail ships like Mercury, which is either undermanned at level 3 or overmanned at level 4 (Constitution is another example). A level 3 captain could start with a brig at 120, then grow into the Mercury @135 midway thru their progression to 150 level 4.
Second, it would add staffing flexibility when adding new ships to the game, since they would be less pressure to staff ships in clusters around the current staffing thresholds.
Third, this would provide a proxy for the player to see improvement in ship performance as they gain XP, ex. reload times, etc. will decrease due to the incremental addition of crew.
Fourth, a player could elect to build or purchase a larger ship that his staffing level currently does not support, knowing that he could use it sooner, rather than have it docked for long periods of time due to having to wait to achieve the next staffing level. For instance, a new level 7 @350 finds the perfect Constitution for sale, but elects not to buy it since he cannot sail it effectively until level 8.