For full-sized continuous harvest, plant 30cm × 30cm. Closer than that (20-25cm) gives smaller leaves but more total biomass — good if you cut weekly for market. 45cm is too wide for the local landrace varieties.
Since your soil is fertile, you can even go to 25cm × 30cm and harvest more frequently. Just make sure to keep nitrogen up — top-dress with diluted CAN or fresh manure tea every 2 weeks.