90 Degree turns are inefficient and not advised, but I've damn sure had to do them before. Ideally with a 90 degree duct adapter, but just bending the duct works just fine. The fan can go anywhere in the line where it fits. I personally prefer to blow from the beginning if I'm carbon filtering in the run, and suck with passive intake from the end of the run otherwise.
Any mathmatical calculation of CFMs (not that I've ever used them) pretty much goes out the window. Just make sure you have a fan that blows "enough."