The self-adhesive silicone rubber tape increases the sticking friction between magnet and surface and thereby counteracts the diminished adhesive force in shear direction.
In general, the rubber tape can double or triple the adhesive force in shear direction. The attainable improvement depends on material combination and temperature. An exact calculation is therefore not possible.
The rubber tape does not work with small or very thin raw magnets. The additional distance of 0,42 mm between magnet and surface may even lead to a lower adhesive force. Too strong magnets, on the other hand, may damage the silicone rubber coating.