Errata (typo).... I wrote:
The numerator is 1 for each non-blank value in B1:B20
because each of B1:B20<"" returns TRUE (1).

The numerator is 0 for each empty cell (or cell with a
null-string value) in B1:B20 because each of B1:20=""
returns FALSE (0).

Correction: because each of B1:B20<"" returns FALSE (0).