I wonder if it's better to treat percentage as a unit or variable. So a-percent x b-percent = c(percent-squared), or c divided by 100 twice.
0.72% ⋅ 0.03%
= 0.72 ⋅ 10⁻² ⋅ 0.03 ⋅ 10⁻²
= 7.2 ⋅ 10⁻³ ⋅ 3 ⋅ 10⁻⁴
= 7.2 ⋅ 3 ⋅ 10⁻³ ⋅ 10⁻⁴
= 21.6 ⋅ 10⁻⁷
= 2.16 ⋅ 10⁻⁶
≈ 2 in 1 million