As a 352x288(240) video will be stretched 2times in its width and height, the borders also will be stretched that much
thats the logic.
So we could recommend:
352x288(240) = Overscan 1
352x576(480) = Overscan = 1 (cause of still half width)
480x576(480) = Overscan = 2
704(720)x576(480) = Overscan = 2 (and if your TV gots a big overscan you even can try Overscan 3
But these values all depend on the specs of your Tv set!
In some cases i.e. you also can use a Overscan =3 at 480x576 ... If your Tv set gots a big overscan /movie cropping