Finding an optimum for a single situation isn't too terribly difficult, the problem would more likely be that the large spread of different situations (maneuvering-wise) that arise. The way I see it:
(WARNING: inconclusive rambling ahead!)
Most combat situations in DW start with one ship or group of ships warping on top of another, be it over a colony, other celestial body, or empty space. In these cases the initial speed of all ships is likely zero. The 'warp pattern' for the attacking ship/ships, in which the ships arrive at a "random" orientation to the target (due to the ring pattern of emergence that keeps the orientation of the arrival vector) and a roughly constant distance, is what makes finding an optimum difficult. If there is one attacking ship, it can arrive with any orientation relative to it's target. Similarly, a fleet will arrive with individual ships having orientations to the target ranging from 0deg (the "back" of the circle, target dead ahead) to 180deg (the "front"/leading edge, with the target behind). Considering the defending fleet changes nothing, as the distance is unchanged and their orientation has no connection to the attacking fleet, again meaning the relative orientations are random. Having a fleet set to, for example, patrol a colony will give the defenders some initial speed and align their ships in the same direction, but this does not change the randomness of the attackers vector and could either benefit or disadvantage them in equal measure.
The closest you could come to an optimum is assuming the middling orientation of 90deg and use the roughly constant arrival distance to the target and somehow (either by in-game testing or developing some kind of formula) determine the ideal ratio of thrusters to engines to reach the target. But there are still two problems muddying this scenario: unless the target is a station, it/they will doubtless maneuver as well - although the average time for a large-ish fleet enveloping the target ship will stay roughly the same, it diminishes the value of that "optimum." Also, your ships do not truly care about how long it takes to get to the target: they just want to reach their preferred weapons range, which adds another variable that changes with each weapon system and weapon improvement.
You try to could base the optimum off of a fight already in progress with many ships in a furball at all orientations. In that case, you might want to know the optimum engine/thruster combination to catch an enemy ship fleeing in one direction. Initial orientation will be again random. If you assume an orientation of 90deg, roughly half your ships in a real situation would do better with more thrusters and less engines - and the other half vice versa - but at least it's a starting point. But then you need to know the distance to the target to find the proper ratio, even assuming 90deg orientation, and that distance will be constantly changing because the enemy ship is fleeing. Additionally, you will have no control over the speed at which the enemy is fleeing - AI ships often have anywhere from 20 to 45 speed. Assuming you can get a real distance-to-target after factoring in speed and turning, you can subtract from that distance your optimum weapons range. Lastly though, you have to factor in your acceleration - and I have no idea off the top of my head what that is based on.
With some math I believe you could figure out the intercept for one combination of orientation, enemy speed, and distance, provided you remove the acceleration issue - though IIRC a little calculus will be required (?). But with so many factors, assumptions, and extenuating circumstances the "optimum ratio" all that work would get you is probably no better than just going with your gut.
Whew. Theory-craft done. Anyone see a (vastly) simpler way to optimize the ratio? =/