John Innes compost is basically 2 parts loam, 1 part humus and 1 part fine grit. My own variation, which I use for most of my potting, is 1 part loam, 1 part leaf mould and 1 part 6mm grit with coarse sand. To this I add bone meal as a fertiliser. I don't try to make my own seed compost because of problems with weed seeds already in the loam and leaf mould.