I made an interesting discovery yesterday, and fleshed it out today. It turns out that, for a given range and a fixed amount of (engine + fuel) there is an optimum that gives the maximum speed. It occurs when the fuel is .2857 of the total tonnage, and the engine is .7143. The engine to fuel ratio is 2.5. I can show how I did this if anyone's interested.
Note that this ignores the gains in fuel economy for a larger engine. I'm still working out how to include those, particularly if multiple engines are involved.
For unitary missile engines the optimum fuel fraction is .2391 and the engine is .7609 with a ratio of 3.182. All of this assumes that you can vary the engine power modifier infinitely, so they don't translate well to very high-speed missiles (or ships). However, it should be of some help for building slower, longer-range weapons.