To determine the tap drill size for metric tapping, subtract the pitch from the nominal diameter for a 75% thread engagement.
For example: Tapping for a small, metric screw that is M2.5 x 0.45 -- subtract 0.45 from 2.5 -- your drill size would be 2.05 mm. An M3.5 x 0.6 tapped hole would require a tap drill size 2.90 mm.