Mikes onto it
If you are not using a barlow or Powermate then M = 1
Standard formula as follows:
f/ration = FL/Ap = 1000/130 = 7.9 (this is the same as the next formula with the M taken as = 1 so is not included)
Or with Powermate
f/ration = EFL/AP
Effective Focal Length (EFL) = FL X M(Barlow or Powermate) = 1000 X 4 (PM) = 4000/130 = 30.7 (It might be simpler to think of the M as a "multiplier" rather than a "magnifier)
magnification without PM or Bar.
magnification (m) = FL/EPFL (Eyepiece Focal Length) = 1000/6mm= 167x
or with PM or Bar
Total magnification = EFL/EPFL = 4000/6 = 667x With an f/ration of f/30.7
Or using a 5x barlow
= 5000/6 = 833x With an f/ration of f/39
It gets more complicated when you add optical elements together. eg. A 2x barlow added to a 4x powermate isn't 6x but 8X