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Hi Darren, you are well on the way with your original post.
Many experienced observers take the view that increments of about 1.6x is ideal.
I am assuming your telescope's focal length is 1200mm. Your sweet spot is likely to be around 100x, which would make a 12mm or 13mm EP about the right mid-point. Assuming you go with the 13mm, then 1.6 longer will give you a 20mm for wider field views, and 1.6 shorter gives you an 8mm EP for closer work.
In my scope, my sweet spot is around the 20mm (125x), and so my war horse is my 20mm Nagler. My wider field EP is the 31mm Nagler, which is 1.55x the focal length. Below the 20mm I use a 13mm Ethos (1.53x shorter), and below that a 9mm Nagler (1.44x shorter again).
One reason that people will often choose not to get EPs in steps of 2x is that you can achieve the same effect with a 2x barlow. If you get EPs in 1.6x steps, then that, in combination with a decent 2x barlow will give you a lot of options.
In terms of what is your upper useful limit, in an 8 inch scope more often that not you will not get to 240x unless the conditions are particularly still. Probably 100-150x is where you will spend most of your time.
Good luck!
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