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Hi Pete, you haven't said what f-ratio your dob/scope is, but assuming it's less than f7, say f6 or f5, I would forget about purchasing a 40mm ep for low power use. The exit pupil will be too large for your eye to accomodate, in other words, you are wasting light your eye can't take in. The longest focal length ep you should consider is 30 or 32mm max.
There's no substitute like getting experience yourself and looking through other scopes and eyepieces to find out what you really like and want. Everyone is different....some like ultra-wide fields, some like super-wide fields, some aren't bothered how wide the view is. Some people zero in on sharpness, again, this varies. How sharp and over what part of the visible field, ie centre, extreme edge. And so on it goes.
The point is, listen to other peoples opinions, but try to find out for yourself what is really important to you. It may take some time and going to public viewing nights/ star parties, etc, but it's worth it. That way, your purchases will stay with you for a long time, and you won't end up with a huge box full of eyepieces, most of which you never want to use.
All the best.
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