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Darren,
One way is to establish the highest and lowest useful magnifications of your scope, and choose eyepieces that span this range. Some can get away with just 3 eyepieces (low, medium, high) but I'll guess that for these people the range between highest/lowest is no more than 5:1.
For more than 3 eyepieces use steps of 1.5X - 2X between focal lengths.
The highest useful power for most reflectors is between 1.2-1.5X per mm of aperture, 240X to 300X for your scope, and at a guess a 4-5mm eyepiece will achieve that.
The lowest useful power is that which gives an exit pupil around 6mm, the eyepiece focal length corresponding to this = 6mm x the focal ratio of the scope. For an f/6 scope, this means a 36mm eyepiece, and your lowest power is 33X.
So... to span this range using 5 eyepieces I'd suggest: 5, 8-9, 12-15, 18-21 and 30-35mm. You could omit the 5mm, the number of times you'll use it in a year will be countable on 1 hand.
For a set of 3, I'd choose 7, 15 and 30mm. Adding a good-quality 1.4X barlow gives you effectively 5, 10 and 21mm which makes a fair bit of sense.
The last problem is how much you're willing to spend vs what sort of eyepieces that will buy, and new vs secondhand. There are some bargains to be had too if you are patient and shop around, and there's nothing wrong with good secondhand ones either.
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