TRial and error my friend. Some plants, Cloud 9 for example will grow so tall and wide that unless you scrog it it will take 3 sq ft per , something like a hash plant you may get by with 18 X 18, mature clones straight to flower in a sog about 1 sq ft, but either way you gotta know your plants and what is most productive for your needs and capabilities. Too many variables to really answer your question