In article ,
"HeyBub" wrote:

Let's apply a test.

A 30x30-foot yard is 900 sq ft. Drilling a hole every 6" (or 6/sq ft) means
you have to drill 5400 holes.

If it's a math test, you flunked. 61 x 61 = 3721 holes. That includes
perimeter holes. Start 3" in from the edges, and it's 60 x 60 = 3600.