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Don't forget the exit pupil
A 40mm eyepiece at f5 will have an 8mm exit pupil. Too big, even for young eyes. Pupil dilation maxes out around 7mm in your teens. Then, it's all downhill from there. The attached chart of pupil dilation is a population average. Individual variations are common.
Aim for something in the low 30's. I have an f5.5 dob and the Nagler 31. Great combo. Now in my late 50's, I have measured my eyes to have an approximate maximum pupil dilation of 5.5 mm. So 31mm is optimum and I don't use anything longer.
31mm Naglers work very well on all scopes so once you have saved up and purchased, will give you a lifetime of use. However not all other eyepieces work well on fast newts. If you can't afford a higher spec eyepiece, you could consider a coma corrector and then you might get reasonable results with a cheaper eyepiece in the 28-35mm range but I can't guarantee this approach will work. Perhaps someone else can comment on this approach.
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