Yes I agree with the math and the principle that bigger more fuel efficient engines is generally cheaper and better from a cost perspective. Compact engines with a high power setting is only useful in short ranged ships. I think this should be common sense.
I often use power settings lower than one for reducing the fuel costs and increase the range of the ships with less space for fuel.
Although when doing your overall calculations you need to include the size the engine and fuel take up since the engine increase in size more than fuel due to maintenance and crew requirements.
One point on the ships above, they do severly lack in maintenance facilities, not that it is important for this comparison though. In general you need at least twice the amount of MSP (military ships) on ships than the most expensive component, one reason I stay away from big engines in the early game not withstanding research cost.
Also, overall fuel efficency of engines change the ratio of engine and fuel. If you have an overall fuel efficency of 50% the ratio of engine to fuel change accordingly for any given range. So as fuel efficency get better and your range requirement stay mostly the same these constraint with engine versus fuel ratio get less and less of an issue.
What I mean to say is that the engine to fuel weight are mainly important on really fast ships where you also want relatively long range. I think that those restrictions are probably not that common in general ship design.
I do however think it is very informative to explain that if you want to give ships exceptional range requirements you need to consider more fuel efficient engines when fuel exceeded a certain weight ratio of the ships engines, anything else is a waste of resources.
In basic terms it comes down to what resources you are willing to make a sacrifice on; build cost, range, fuel consumption, research cost, speed, mission tonnage, uppgrade/refitt issues and so on.
If we take Magneto Plasma tech as an example my military ships would likely have speeds varying from 2500-5000 km/s depending on role in the fleet. I would deem 4000km/s as a fast ship the likes of a Destroyer. Engines would range from 1.25 down to perhaps 0.75 in power efficiency depending on my needs, fuel consumption is allways an important issue, especially when you have lots of fighters and other parasite ships that have extreme fuel usage during operations.
I also invest as much research into fuel efficiency as I can, that is generally more efficient the first four or five engine generations than building big research costly engines. In principal my ship engines up until Magneto Plasma are size five, just don't like size one engines on real ships, not a rational thing though..
In general I have no problem sacrificing speed for fuel efficiency, mission tonnage capacity and retrofit expediency on some types of military ships, generally the really big ships such as carriers. Ships wich are not suppose to be even remotely close with the enemy, or in ships that need both mission space and range, such as long range cruisers who operate alone or in small packs far from friendly bases. I often also try to target ships to a certain fuel efficiency, lower and it is OK with a more powerful engine and either I get more speed or more mission tonnage but also need extra fuel, but no point if it does not give me anything in return. The list goes on and on about what sacrifices I need to do in every design.
Engine to fuel efficency IS an important part of this process. It is certainly good to know when your designs start to waste resources.