Shear loads are due to the holding force of the screw. A screw is usually applied to hold something else. This causes some loads on the screw, which are for bosses usually axial to the screw and therefore cause shear on the inner surface of the boss.
This load on the screw is transmitted via the thread onto the boss. Because of the large Young modulus of the metal in comparison with the polymer we can assume that the load is distributed evenly over the inner surface of the boss.
To get the value of the shear load we first have to get the torsion force of the screw. The torsion itself was set to 1.198 Nm.
We assume, that the radius on which the torsion force is applied is half of the radial engagement of the screw in the material. With a pilot hole diameter of 2.79 mm and a outer screw diameter of 3.7 mm this gives us a half engagement diameter of 3.25 mm, so the radius is 1.62 mm or 0.00162 m.
Together with the torsion of 1.198 Nm this gives us a final torsion force of Ft = 739.5 N.
A few simple trigonometric calculations give us the correlation between the torsion force and the shear force, depending on the trailing flank of the screw, which is (=30( for the selected 6-22 HiLo Plus screw. The Shear force is Fs = 320.2 N.
It is assumed, that this shear force is distributed evenly across the whole inner surface up to the length of engagement.