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The 5,6 & 7mm focal lengths are ideal for 8" f6 and planetary viewing giving 240x, 200x & 171x with 1200mm focal length and exit pupils of 0.83, 1 & 1.17mm at f6.
You will find the 7mm very often used on nights when seeing is above average but not exceptional. 171x gives a good image size on Jupiter and Saturn. In fact 8mm would be useful too giving 150x which is still ample for the larger planets and provides for a sharper view when seeing is average which happens quite a bit. (120x too is great on Jupiter).
If you want more power then get a quality 2x Barlow and you will have 8, 7, 6, 5, 4, 3.5, 3 & 2.5mm but the shorter focal lengths are more suited for a short refractor and will give a smaller exit pupil at f6. Start without the Barlow though as you may find 5,6 & 7 is all you need.
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