I also usually use 50% of the missile for engines and around a third for warhead. At least that is it for casual asms.
At higher tech or great ordnance and military volumes, I might shift to bulky size 12 torpedoes who have one maximum possible 5MSP engine for best range. The reason is that later levels of ecms make them good enough to deal with technologically inept races and some spoilers, so you can dispatch an enemy ship ideally with 1 or just few torpedoes each. It is poor tactic against well equipped enemies though, and just serves to take the micromanagement out of the game where it is not necessary anymore.
I also use maximum power grade available for straightforward asms, but I usually also design a x4 power factor one to get capacity for real "cruise" missiles, or tactical torpedoes, or however you want to call them. Having one ammunition type that can really go the distance is sometimes of great strategic value, for example when closing in is just too risky. I also prefer those engines in any case (or even just x3 at earlier tech) when making m.i.r.v.-s, because these clearly don't need speed to beat any targeting check.