I sow Nomocharis much as Maggi and Ian do but never have enough seed for poly boxes. I use a pot size - for anything - appropriate to the number of seeds available, usually a 4" or 5" square pot. I use my regular potting mix as I do for all seeds, removing any lumpy bits from the surface and for cool-loving plants or bulbs add some crumbled leaf mould, pea straw or pine duff. Seed on top of this then cover with course grit. Pots remain outside at all times and are watered daily at this time of year (summer for me). Nomocharis usually flower in 3 years from seed here. I find them no more difficult to grow or flower than say lilies or the cooler frits or meconopsis and primulas, given that all these things like a cooler, moister climate than what is natural here, so watering is essential while such things are in growth.
