The basic rule is if you are going to grow a ‘traditional’ orchard, then whatever the ultimate height of your tree, will then also be your planting distance. For example M26 will grow to around 3 metres and so will need a planting distance of 3 metres, M27 will grow 1.8 metres and will need a planting distance of 1.8 metres.
But if you are growing your trees using a different training method ie cordons, then spacing is reduced. Something like 60cms between trees and 1.8m between rows is appropriate.