Dear Sebastian,
Your equation for the tip-to-tower clearance is not correct. The precone and shaft tilt angles should sum. As discussed in my Jan 18, 2013 post in the following forum topic: Tower-blade tip clearance!, the clearance for the undeflected NREL 5-MW wind turbine (not including subtracting the tower radius) equals: (5.019 m)*COS(5 deg) + (63 m)*SIN(5+2.5 deg) = 13.223 m.
Best regards,