[PAR_WIG1350]

Spoiler for :

theta=arctan(6/f2)
-------------------
mu=n(theta)+.2
.8=n(pi/2)+.2
.6=n(pi/2)
1.2=n(pi)
1.2/pi=n
mu=(1.2/pi)theta+.2
mu=(1.2arctan(6/f2))/pi+.2
----------------------------
f2^2+36=(10(mu))^2
Sqrt[f2^2+36]=10(mu)
Sqrt[f2^2+36]/10=mu
-----------------------

Spoiler for :

Sqtr[f2^2+36]/10=(1.2arctan(6/f2))/pi+.2
Sqrt[f2^2+36]pi=12arctan(6/f2)+2pi
Sqrt[f2^2+36]-12arctan(6/f2)=2pi
that's ugly
Wolfram Alpha couldn't make it look any better, but it gives us an approximation for f2 of about

Spoiler for :

10.8N