To be honest if your a new grower I,d stay with premixed soil . Your not going to save money mixing your own right off the bat . It costs more to buy all the individual ingrediants for a mix . Then if its a problematic mix you created being unexperianced you won't know whats going on .
Staying with a known good soil will give you a chance to read your plants and learn what they like , then later down the road mix your own at that point you want have to ask for a mix you will already know what they want. I was gung ho on making my own soil at first , then I learned there are so many diff recipes everybody's is different .
Happy Frog with extra pearlite ,dolomite lime , and kelp meal has been working good for me .The only advantage to a mix is one like Subcools Supersoil mix because it's a water only mix if you can manage to get it right . He has never posted an accurate ratio for making small batches.