Hi mate, all depends what you are growing. Autos or photos. I started at the lower end with BioGrow, 1g/ltr. Bio bloom 2g/Ltd but I think I over did it on the bio bloom.
I'd start with only 1g/ltr as my plants were happy with that. But if you plan a long veg you may need to up that. The dose is supposed to last eight weeks. But I think it's more like 6 with 1g. Hope this helps and good luck