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Hi John.
Magnification is determined by the focal length of your scope, divided by the focal length of your eyepiece..
eg: your 12" dob is 1500mm FL I think.. So if you have a 30mm eyepiece, the magnification is 1500 / 30 = 50x
A 20mm eyepiece gives you 1500 / 20 = 75x
A 2x barlow lens effectively doubles the focal length of your telescope, so instead of 1500mm FL, it's 3000mm FL. So the magnification of your eyepieces is also doubled.
eg: Your 30mm eyepiece now becomes 3000/30 = 100x
and your 20mm eyepieces now becomes 3000/20 = 150x
However most people talk about it in terms of eyepiece focal length because it's easier to remember that way.
So a 30mm eyepiece with a barlow, effectively becomes a 15mm eyepiece. The 20mm eyepiece with a 2x barlow, effectively becomes a 10mm eyepeice.
So that's why people say, "don't double up". That is, if you're getting a 2x barlow, don't get a 30mm and a 15mm eyepiece, because it'll mean you're doubling up on the 15mm focal length.
Choose focal lengths that give you a broad range of magnifications, with and without the barlow.
Does that help?
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