How are myopia and astigmatism calculated?
Are they calculated together or separately?
I believe that they are calculated together. So if both components are minus (-) then they are added to get the total refractive error.
From the JAA website (i.e the generic Jar-FCL 3, your country might have modifications):
JAR–FCL 3.220 Visual requirements
(b) Refractive errors. Refractive error is defined as the deviation from emmetropia measured in dioptres in the most ametropic meridian.
Cosmo