Firstly, keep the chicken call off, it's not natural, secondly take a look around the farm in daylight to see if you can find any paths prints etc,
What is the farmer having problems with lambs, hens,
early morning or after 12 till 2 am i find best, As suggested use a stink pit for a while and stay clear after a week or so till the fox builds up it's confidence in comming to the same area. DON'T light up the whole sky when lamping keep the light low and slow rotation combing over the land.
Stay downwind if at all possible on sight of a fox.
100 yards shot or there abouts with hmr. not an