First u must calc TRUE ABILITY for engine

TA = %(as in sailor "vetting" window - add all three %) x TotalMAXSailors x Ab.Displayed

then

for 1 engy

OHTime = Natural OHTime (as in Engine Tables) + TA /110.000

for n engies

OHTime = Natural OHTime + (TA1 + TA2 + ... + TAn) /110.000

The above can be see it as every 110.000 in any of your engies u gain 1 second for OHT. For example, in ur comparison pair, two 120 eng with 914 sailors max., one base 12 and second elite

TA1 = % x 914 x 3398

TA2 = % x 914 x 3543

Asuming equal %

TA2 - TA1= (% x 914 x 3398) - (% x 914 x 3543) = % x 914 x (3398 - 3543) =

= % x 914 x 145

so for a %=136% (109 Vets|805 exp |0 rookies) u have

TA2 - TA1= 136% x 914 x 145 = 180.240 or 1.63855 seconds more with elite one that 12 Base one

Obviously a sailor Base 12/11 Eng/rep is superior as a whole to an elite Engy 13/9 (slightly less eng but much high rep) so if u has one of these keep it!