Note: on tapping holes:
I normally specify a tapped hole depth of 1.5 dia for a fully threaded hole in a blind tapped hole (steel male and female).
The depth of threaded hole if a plug tap is not used i.e for automatic one stage tapping would be deeper.
According to BS1936 the tapping drill should be have a depth greater than the tapped depth (clearance depth) as follows
Less than M6 = 0.25 x full thread depth...
M6 TO M12 = 0.2 x full thread depth...
Greater than M12 = 0.15 x full thread depth.