There are two radii in the side of a field, so the square formula becomes (2r)^2.

Now the ratio is πr^2 / (2r)^2

Distribute the square and it becomes πr^2 / 4r^2

Now the r^2 cancels leaving us with a constant!!

π / 4

What does that constant mean? Well if we were leaving the corners of our field for the poor, π / 4 is how much of our produce we would be taking home. So how much does that leave for charity?

1 - π / 4

Okay great, so I just need to give that much charity. So lets say Sally makes 60K a year after taxes, how much does she need to give to live up to the commandments?

(1 - π / 4) * $60,000 = $12,876.11

Also, I didn’t do the math, but you can probably knock ~1K off that because our government spends your tax money on social programs.