Darren,
You can easily measure the negative focal length of your barlow, then use that to calculate, using it's position relative to the prime focus, to give the position of final focus and the amplification factor.
The barlow focal length:
Measure the free diameter of the barlow lens (say 30mm)
Position a card with a circle twice the diameter (ie 60mm) and project the sun until it fills the circle.
The distance from the card to the lens is the focal length.
Hope this helps.
ken
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