Designing effective missiles is both a craft and an art. I am offering this small analysis in hope that other Aurora noobs (like myself) might benefit from it.
This has probably all been done before, but Search didn't turn up much and the article in the Wiki didn't mention any of this.
Your missile to-hit chance depends on the speed and agility of the missile, and on the speed of the target. For anti-Ship missiles, a 100% to-Hit vs speed 10,000 kps targets is probably as much as you'll ever need... at least, in the early-to-mid game... but for anti-Missile (PD) missiles, the higher the to-Hit, the better; since enemy missiles might move at 50,000 kps or more.
A complicating factor is the rounding that takes place with both Speed and Agility. Capt Kiwi and I have been able to figure out the Agility rounding, but the Speed rounding is still a minor mystery... so we can offer only a partial solution to the missile design problem.
After allocating enough Warhead to get the "bang" you want, and allocating enough fuel to get the range you want, you will be left with a certain number of MSP to be distributed between Engine (increasing your speed) and Agility. Both contribute to your final to-Hit capability, but not equally.
These MSP should first be distributed as follows:
If S = the missile size,
and M = your current missile agility per MSP (depends on tech),
and T = the total MSP available for Engines plus Agility,
Then E = (T/2)+(5xS/M) is the amount of MSP that should be used for Engines
and A = (T/2)-(5xS/M) is the remaining MSP that should be used for Agility.
These numbers should then be adjusted up or down so that your actual missile agility is a whole number... ideally, a whole number that is an even multiple of your missile size... like so:
This should give you something approaching an ideal design for that size of missile... although if it is a ship-killing missile (instead of a PD missile) and your to-Hit greatly exceeds 100% vs a 10,000 kps target, you might want to transfer more MSP from Agility to Engine to increase the speed at the expense of superfluous to-Hit. To-Hit numbers above 100% aren't much use against ships, while Speed is always useful to avoid the defender's PD fire.
Note that this solution is only approximate, since it takes no account of the engine/missile-size/speed rounding.
Thanks to Capt Kiwi for the basic continuous-function formula.