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.

You could also replace % with 10⁻² and use scientific notation:

      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

It's because we usually dont multiply percentages, but instead work with normal decimal figures.