This calculator will find the distance you need from the pivot of a circle, towards the perimeter, to find the desired acres for partial half circle planting.
Enter a distance from the pivot, I'll tell you the acres in that area from the pivot, then the acres in the remaining area out to the circle perimeter.

Circle Irrigated Radius in Feet
Distance From Pivot To Calculate
            

Enter values into the two form boxes above. The radius will stay populated with your last entered value. Enter a new new number if you like.