Well after see some posts about true ability,  I use the next formula:

Ta=[(4Nv/Ns+Ne/Ns)+(Na/Ns-1)]Ns·At 

where:
Ta: true ability
Nv: Number of vets
Ne: Number of experts
Ns: Max possible current number of sailors
Na: Number of active sailors (Na=Number of recruits+Ne+Nv)
At: Displayed ability of the sailor (Sum of the stat from the start at the last lvl) 

Well this work well for full filled sailors, but with some Bo the penalization due the lack of recruits doesn't work like the next example:

Mi BO :
Na=472
Ns=782 
At=2257

The game shows a rate of -46% of ability, but using the part of the formula who gives me a -39%, with other bo works, could someone give me a help :). Thanks for your time guys!.