I have several zooms and I'm with clearwaterdave: the view matters more than the magnification. So I adjust the focal length until I get the view I want (best framing or sharpest view the seeing will allow etc) and never worry about what the actual setting is.
The only time I ever actually need to know is when I am logging things for my Astroleague observing projects and then I look at the numbers on the scale and guesstimate if I need to.
I don't know any reason why you would ever need to know the exact focal length. However if for some reason you ever did, you could try the method TOMDEY outlined in this thread:
It would require that you have previously calibrated the EP of course, as the OP in that thread was looking to do.
Again, like clearwaterdave said, use it, enjoy it and don't stress out over something that is not important.