Besides impulse speed and starship maneuver skill there are other factors involved as well. Escorts get 10% more bonus defense, having the elusive trait gives 10% more, and using the Aegis engines gives 5% more, with an additional 5% more from the 2 piece set bonus.
You can have a max of 90% if using an escort with elusive and max maneuver skill while going at impulse speed 24 and using Aegis set.
Does anyone happen to know how a ship determines what it's maximum bonus defense is by chance? My google-foo failed and servers are all down =(
There are a number of factors:-
Obviously there's speed.
There's the elusive trait (+10 bonus defense)
Escorts have an innate +10
AEGIS engines have +5
AEGIS 2-piece set gives you a further +5
Attack Pattern Omega
As for the actual formula I'm afraid I have no idea.