Shovel your soil out and then read the ingrediants of many of the potting soils sold in the big box stores such as Home Depot, Menard's or Lowes, you will see that most of them are a decently close approximation of "Mel's Mix", except for the compost. One thing that I believe Mel missed, is that it's good to have some of your own soil mixed in with the mix...approx 15% by volume. Plant roots don't like to grow into one type of soil (Mel's Mix) and then another completly different type (your soil). Check with your Extension Service to verify this. So, for the 33% compost, use 15% your soil and the rest different types of compost.

For 900 square feet (450 cu ft @ 6") you need a "lot" of Mels Mix. Forget it on that scale and find a truckload(s) of manure. Or use Mels Mix and pay about $3-4K.

My first raised beds were last year, so I'm no expert yet on square foot gardening. I followed the instructions for Mel's mix as closely as I could, and my plants grew just fine. Better than fine, actually. That being said, I believe that you can have growing success in a wide variety of growing mediums, as long as the nutrients are there, and the soil handles water properly. I have a suspicion that what you have so far would be fine for this year. I'd go ahead and plant, and as the plants are harvested at the end of the growing season, there will be room for more of the ingredients to make the soil more like Mel's mix. Just my opinion.
Grant