That’s the “minor” diameter of the screw. If you want to be ultra precise you can look up thread measurements online based on the standard size of the hardware (eg #5 wood screw is 1/8 basic size, .085 root or minor). You don’t need anything to strong as the bolt is not supporting the weight of the desk. Only keeping the leg from moving laterally. The weight of the desk will be resting on the leg and you could just set it there with no hardware temporarily, and it would stay there.