Henry,

I think I see what you are saying. The mark you make will lie on a line which will end up in the center of the field, but it won't usually be itself in the geometric center of the field. Assuming the placement of the two marks B and C is absolutely precise, it is simple to do the calculation and for an error of 1/6 inch in 25 feet, I get about 5 seconds error in the angle. But since the angle need not be horizontal, the deviation of the line on the ground from perpendicularity would be less. As I noted this ignores any errors in the placement of the marks B and C. I could analyze that also, but my guess is that it wouldn't materially change the result. The deviation from perpendicularity would be negligible.