Sorry to be an idiot – I did the math, but I don’t trust myself, and I can’t find an online converter to check it against.
I need the acreage equivalent of a circle that’s five miles in diameter. Area is Pi r[super]2[/super], so pi x5[super]2[/super], or pi x 25, or roughly 78.5 square miles, which would be a tad over 50K acres.
If the radius of the circle was five miles, your first calculation was OK. (I kept an additional decimal, so I got 78.54.)
Your self-check for that is also pretty easy. A radius of 5 is a diameter of 10, of course. A circle with a diameter of 10 would fit into a square of 10 units to each side with a tangent on the mid-point of every 10 unit side line segment. A square that is 10 x 10 would be 100 square units, so a circle of 78.54 units for a square with the corners knocked off seems realistic.