Your plant will absorb nutrients best when ppm levels coincide with the size. When you have a seedling you want to keep the ppm's at low 3-500. This would be the first two weeks of nutrients. On the 21st day of growth you should boost your ppms higher to about 8-1200 like your mate said. Keep them their until after a month of 12/12. Then you can go up to 12-1500 because at this point your root zone should be large enough. The last two weeks before flush you can even push your ppms up to 18-2000. At this stage your babies are so far into maturing your plant won't die, just absorb nutes and grow fat trichs. I really don't think you should push your ppms that high if you havn't got much experience. Hope I answered your question.

Plant size = ppm levels
small plant - low ppms
large plant - higher ppms