[Wavytone]

Timing a star is the only reliable way. Do it first without the Barlow, then insert Barlow and repeat.

If all else is the same (star,eyepiece and scope), the ratio of the times will give you the actual power of the Barlow directly without any further calculations or assumptions regarding focal length. Very simple and accurate.

And yes the power does vary depending on how far back the eyepiece focal plane is located from the Barlow lens. Whether this really matters ... I don't think so.

But what it does mean is that a Barlow chosen from brand X is highly unlikely to be optimal for eyepiece brand Y, in the sense that the designer of one has no control over what you're going to mate it with.

The best results should be from a Barlow specifically designed to match your eyepiece (there are some, for example Edmund had one for the RKE's, TV have the Powermates presumably for their eyepieces, and Vixen made one for the LV series, and Clave did too).

But... Also note that many modern eyepieces actually have a Barlow built-in as part of the eyepiece design, specifically designed to compensate the aberrations from the rest of the eyepiece - this is the trick in the Vixen eyepieces, Hyperions, TV Naglers, ES ultrawides, and many more. Sticking another (unmatched) Barlow in front isn't going to be as good as using the right eyepiece with the right focal length. It's called a Smythe lens (after the inventor). The thing is, with the exception of the Hyperions, the eyepiece won't work well with the Smythe lens removed, so the eyepieces aren't designed to let you remove it in the first place.

Barlows were a pretty common item back in the 19th century. It was common knowledge that putting a Barlow in front of a low power Plossl, Orthoscopic, or Erfle made a pretty good high-power eyepiece with good eye relief. Smythe took the obvious step to permanently mate the Barlow in the eyepiece, then re-optimise the overall design to reduce aberrations. The only snag in Smythes era was the lack of antireflection coatings at that time, so while it was a great idea, it never became reality as there were too many air-glass surfaces - light losses and reflections made a practical eyepiece impossible.

When modern good multilayer dielectric AR coatings became standard in consumer-grade eyepieces in the 1980s... voila, Masuyama, Vixen, Al Nagler all jumped on board and we have modern ultrawide eyepieces with long eye relief, virtually all stemming from Smythe's idea.