The "optical purity" is a comparison of the optical rotation of a pure sample of unknown stereochemistry versus the optical rotation of a sample of pure enantiomer. It is expressed as a percentage. If the sample only rotates plane-polarized light half as much as expected, the optical purity is 50%.

$\%$ optical purity $=\frac{\mathrm{observed} \mathrm{rotation} \mathrm{of} mixture\times 100}{\mathrm{rotation} \mathrm{of} \mathrm{pure} enantiomer}$

$=\frac{+12.6°}{+30°}\times 100=42$