Field strength at a distance can be loosely calculated but there is not necessarily a correlation between physical pull strength and field strength.

Whilst the physical size of a magnet will have a bearing on the field strength it is more pertinent to consider the shape of the magnet.

For example, a rod magnet that is longer than it is wide will measure far more (Gauss) than a thin disc magnet.

You can use the following formula as an indication of field strength at a given distance:

Br/2 (L+X/ √R²+(L+X²) – X/√R²+X²)

Where Br is 13,200 (N42 magnets)

L is the thickness of the disc magnet (mm)

R is the radius (mm)

X is the distance from the Pole face (mm)

I hope this is helpful and please let me know if you need any further information.