Disclosed is an assembly for a dosing system of a spreader machine for distributing a granular material or the like, which comprises first and second members provided with first and second openings, respectively, and an actuator connected to at least one of the first and second members and configured to provide a driving force between the first and second members, thereby, adjusting an opening size of a dosing opening provided by an overlapping area between the first and second openings. The first and second members are moveable by relative movement into a first position providing the dosing opening with a first opening size and a second position, different from the first position, providing the dosing opening with a second opening size different from the first opening size, wherein a center portion of the dosing opening is locally fixed when the members are moved between the first and second positions.
