If any use, this is a pretty good summary of the formulas you could use to calculate magnification for conventional Barlows. It also deals with the telecentric ones (like the TV Powermates), that don't have much(or any) variation in magnifcation.
http://www.brayebrookobservatory.org...BarlowLens.pdf
Practical data can be found on the Televue site, which has some useful tables that give magnification factors for their Barlows.
https://www.televue.com/engine/TV3b_...=52&Tab=_photo