This is propably just basic math that i have forgotten, but how do you calculate the directional area (or seeing area) of an object? Let's say enemy sight in degrees of it's front.
I could never figure out the math for that. There was always one extra detail that would give me a headache.
Just use the Advanced Direction Object. It has a function for determining the direction between two points, and one for determining the difference between to directions. So you find the direction between the seer and its target, and determine the difference between that and the seer's angle. If it's less than its range of vision, then it's seen.
Also, I believe CF2.5 has an "angle of a vector" function built in which you can use, but it's nice to use the Advanced Direction Object's "difference between two angles" anyway, because you don't have to worry about weird cases like crossing from 360 degrees to 0 degrees, or dealing with negative angles, or angles higher than 360 degrees. It just simplifies things.