Refer to figure.
The ICAO VFR semicircular rule of flight level and/or altitude is based on the magnetic tracks:

• For magnetic tracks between 0° and 179° (easterly magnetic tracks), the flight level or altitude must be odd + 500 ft (e.g. 3500 ft, 5500 ft etc.).
• For magnetic tracks between 180° and 359° (westerly magnetic tracks), the flight level or altitude must be even + 500 ft (e.g. 2500 ft, 4500 ft etc.).

The purpose of this rule is to limtit possible conflicts between other aircraft flying VFR coming in opposite direction by providing 1000 ft vertical separation between opposite west/east tracks.