How can we calculate magnetic declination or variation at any given place?

One way would be to use magnetic declination or variation values provided on maps. Unfortunately, declination values are constantly changing.

When printed maps were the only way of getting this information, the declination values were somewhat out of date by the time the maps got to the general public. The diagram to the right shows a negative magnetic variation value of about 7°W for an Ordnance Survey map of Purbeck, UK in 1977. The value in 2011 was a little over 2° W.

Another way would be to perform a prediction. This web page operated by the National Geophysical Data Center (NDGC) offers a fairly good value for declination. The model reflects a highly predictable rate of change, and will usually be more accurate than a map, and almost never less accurate.

Magnetic charts from the USGS National Geomagnetism Program give an indication of the complexity of the global pattern of magnetic declination values.

Ordnance Survey map showing Grid North, Magnetic North and True North variations for Purbeck UK, 1977

A better way however is to use this https://setcompass.com website which computes declination for any place on Earth using an algorithm that implements the World Magnetic Model. An example for the Aircraft Graveyard in Tuscon is shown below where the declination is computed as 10º 28' 48" E (10.48º).

Tucson Aircraft Graveyard: positive magnetic variation or declination of 10° 28' 48" (10.48°)

Tucson Aircraft Graveyard: positive magnetic variation or declination of approximately 10.5°

Users of magnetic-field models and charts must be aware of their limitations. The Earth's magnetic field is extremely complicated, in both space and time, and magnetic-field models are, by practical necessity, something of an approximation of the actual magnetic field.