The following explanation is from PitturiAmo at https://www.pitturiamo.eu/come-vendere-quadri-online-articoli/il-coefficiente-di-un-artista-come-valutarlo   (cf. English translation by Google)

The coefficient of an artist is an indispensable tool for the correct evaluation of a work of art. It is issued with official certification by art galleries, and allows an artist to obtain tangible recognition for his art by critics, gallery owners, curators, and art professionals in general.

The evaluation takes place by means of an official certificate in which the so-called "Artist Coefficient" is defined. It is a numerical value, which is determined and agreed upon by the gallerist together with the artist. The Coefficient corresponds to the level of fame and artistic quality of the artist, and attests to it at the national level.

The formula, which is used to value a work of art from its size and the artist's coefficient, is as follows:
    V  =  (W + H) × C × 10

that is:
    [ Value = (Width + Height) × Coefficient × 10 ]

Example: a painting with dimensions of 100 × 80 cm, with coefficient 2:
    (100 + 80) × 2 × 10  =  € 3,600

In this example, the base value will be € 3,600. (dimensions are in centimeters, value in Euros)

Also see the calculator at https://www.artingout.com/calcolo-quotazione-arte   (cf. English translation by Google)