I made my own seedling potting mix. It has 35% drainage and 65% water hold matter. Now, that I see it, I feel like it has too much drainage. I used roughly the same percentage of as Sungro's Blackgold Seedling Potting Mix. I have used Blackgold with great success in the past.
I know there are many potting mix formulas out there. I know everyone has their own preferences. So, I want to focus on industry standards. What is the industry standard of Inorganic:Organic Matter in seedling potting mix?