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!