ATM Questions on Magnification

[Barbara & Matt <B.Smerald@worldnet.att.net>]

Hello,

  Can anyone on the List clarify the following discrepancy?

  If one has a 'scope of 60 inch focal length and an eyepiece of 5mm  
 focal length with a 68 degree apparent field, the standard formula for
 finding the magnification (PrimaryFL*25.4/EyepieceFL) yields a 
 magnification of 304.8X - with a true field of .223 degrees, while the
 proportedly more accurate formula for finding magnification, using the
 tangents of True and Apparent fields, yields a magnification of 346.45,
 even though the True and Apparent fields seem to remain the same in
both cases? 
 
  Which is the more exact way of determining magnification - using
 focal lengths or fields? If fields, how can eyepiece focal length
 be exactly determined given both True and Apparent fields and Primary
 focal length and, conversely, how can the Primary's focal length be
 (again, exactly) found using True and Apparent fields and ocular focal
length?
                                  
                                           Matt