true ability = ( ( (4 * veterans number) + experts number) * ( (total crew / max crew) * displayed ability) ) / 1mil
total crew = the number of rookies/experts/vets (for example, trimmed torpers/gunners - sailor which is not fully filled; or if fully filled, the number will be the same as "max crew")
max crew = the maximum number of rookies/experts/vets sailor can hold.
In order to calculate total crew, you have to rookies+experts+vets, while max crew is shown.
Originally Posted by http://nf-guides.com/sailors.htm
check this screenshot: http://nf-guides.com/Sailors/skill_nation.jpg
{D} - stands for max crew (number will be always shown, no calculation needed here),
for total crew you need to {A}+{B}+{C}.
this formula is the same for all abilities, but for some calculations this true ability formula is not enough. For example spread calculation, so you calculate true ability and put it in that spread formula.
Credits:
Aldabert for his true ability formula,
and who ever worked on www.nf-guides.com