Drag the marker and get values of Grid Convergence and Magnetic Declination for that location.

Grid Convergence is defined as the difference between Grid North and True North for UTM projection (East Convergence is positive).

Magnetic Declination is defined as the difference between Magnetic North and True North (East Declination is positive).

Grid Magnetic Angle is defined as the difference between Magnetic North and Grid North (East Angle is positive) and it is calculated as 'Magnetic Declination' minus 'Grid Convergence'.

If you are using OpenOrienteering Mapper insert Magnetic Declination value in Mapper 'Declination'.
Mapper 'Grivation' is automatically calculated by Mapper.

If you are using OCAD you should consider a switch to OpenOrienteering Mapper, but if you really want to stick with it just insert Grid Magnetic Angle value in 'Real World Coordinates - Angle'.

This is meant for UTM georeferenced maps only, but it works also for other Transverse Mercator projections having the same central meridian as their UTM equivalent. So it works for instance for GaussBoaga Italy West - given that you are inside UTM32 or for Slovenia D48 (inside UTM33).

Viewer made by Remo Madella (Rem Maps).
Huge inspiration from Måns Beckman at
Grid declination is taken from geodesy.
Magnetic declination is taken from geomagJS with data from NOAA's World Magnetic Model - WMM2020 update.