How edit a missile to guarantee it always finds its target

Add the heat seeker feature to it. In the ini file under weps in the equip folder, find the missile you want and add

HP_Trail_Parent = HPExhaust
seeker = Lock
time_to_lock = 0
seeker_range = 1000
seeker_fov_deg = 35

You can change the range to longer or shorter

These are the lines you want to change:

seeker_range = 1000 How far away does the target have to be in order for the missile to lock on (in meters).
seeker_fov_deg = 25 The angle at witch the missile will seek the target. Ex: setting it to 180 will mean it will seek a target all the way to your right or left, while setting it to 360 means it will seek a target no matter wear it is.
max_angular_velocity = 2 This is the important one, i'm not exactly sure what this means, but i think it's the maximum turn angle. All i know for sure is setting it to 90 will allow the missile to make any turn.

accel = 25.000000 Make sure this value under the engine section isn't to high, otherwise the missile will never make the turn, no matter what you set the above value to (in k/s).

And finally:
lifetime = 12.500000 This determines how far the missile flies before it dissapears (in kilometers).

max_angular_velocity = 2

Try increasing that value significantly

You need the heat seeker aspect of it, it gives the missile a "Target" to lock on to.

if the seeker fov degree goes over the range the missle lancher can on the ship turn it won't fire just keep it around ~80 and shoot when the ship is in front of you

I have actually made this myself. The attached file is a working mod (using FLMM) that has a few options added into it.[attachment=0]<!-- ia0 -->Novastalker Mod.zip.flmod<!-- ia0 -->[/attachment]
The critical changes are simply thus:

[Munition]
...
seeker_fov_deg = 180
max_angular_velocity = 10.240000
...

Don't ask me where 10.24 came from, I really don't know. You can set that to whatever you want.
But the units for max_angular_velocity are rad/s or radians per second. If you don't know what radians are, they're identical to degrees except there's 2p (6.2831...) in a full circle instead of 360.

After running some tests on what exactly is the best seeker_fov_deg value, it appears as if the maximum effective value is exactly 180. Even when set higher the missile won't find anything more than 90º off its front.

Have you downloaded the SDK?

All the inis are normally BINI encoded. You can decode them with FLE, but its a better idea to use the SDK, which ist just a packacke with all inis decoded.