The bolt seems to be an M6 bolt since I measured it as having a 6 mm shaft diameter.
I recommend you use a Bolt Preload Calculator such as this one:
Bolts come in a variety of material strengths. In the image above, I have selected a Grade 10.9 material which has a yield strength of 900 MPa.
You have assigned Structural Steel with a yield strength of 250 MPa to the bolt. Change the material yield strength of the bolt to a higher value and the bolt equivalent stress will be less than the new higher yield strength.