What I do is first I decide how big the missile will be. Anti-missile missiles are always size 1 in my book, while I tend to make size 4-8 anti-ship missiles. Next, I decide how much warhead I need. AMM's only ever need 1WH, since missiles almost never have armor. ASM's tend to be a square number, since that provides the best penetration for their size. for instance, WH4 missiles will damage 3 tiles on the first layer, and 1 on the second. WH9 damages 5 on the first, 3 on the second, and 1 on the third. So now I know how much bang I need, but how much msp do I have to spend on WH to get that bang? Look at your warhead tech level. Each tech level provides a multiplier on warhead strength, such that warhead tech level 10 provides 10 times what you spend on warhead msp. So if I spend 1 msp on warhead, I get a missile with WH10. I'm not sure exactly how the rounding works for fractional WH values, but this provides a good enough estimate. So lets say I'm making an AMM. I know I have warhead tech 10, and need WH1. So I provide 0.1 msp to warhead. Next I make a really powerful engine, since this is going to be an AMM. Speed is everything with these. I might make a 0.7msp engine, with a pretty high power modifier. For ASM's I might use a similar percentage of engine mass, but on a lower power modifier, to save fuel. Next, I find how much fuel it needs to reach the desired range, which is based on the fire control that'll be firing this missile. For AMM's that usually ends up around 0.05 msp. Then, I spend the last of my 0.15 msp on agility. This might decrease the range a little, so I might go down to 0.14 msp to agility and bump the fuel up to 0.06.
Your AMM shoots further than your ASM, which is pretty bad. Basically it means you could block their missiles at 44m km, but couldn't shoot back till they close to 33m km. Both missiles are pretty slow too, I recommend using a higher power modifier, and researching better fuel efficiency.