The right drill for every metric, UNC and UNF thread, for cutting and forming taps. Choose how much thread you want, see whether the hole stays within the 6H tolerance, and get the depth to drill for a blind hole.
Percent of thread compares the depth actually cut with the full basic thread depth (1.299 × pitch on the diameter). Cutting taps remove material, so the drill is the minor diameter. Forming (roll) taps push metal up into the crests, so they need a larger drill.
TD1 (6H) from ISO 965-1. For cutting taps the drilled hole should land inside the 6H range, or the thread gauge will not pass. Engagement length: about 1 × D in steel, 1.5 × D in cast iron and 2 × D in aluminium for a joint as strong as the bolt.