The opening rate is related to the way of hole layout, hole diameter and hole spacing.
We assume the hole diameter is 1.8mm, the hole spacing is 2.8mm.
When you use square array screen sieve:
The opening rate= Single Hole Area / Square Area Between Holes:
π/4×1.82 / 2.82
=0.3246
=32.46%
When you use equilateral triangular array screen sieve:
The opening rate= Half hole area / triangle area between hole spacing:
π/2×1.82 / (2.82×√3)
=0.3748
=37.48%