First, ditch the Miracle Grow products. I could give you a dozen very good reasons not to use it, but the most compelling is that it simply does not work well on cannabis. Get something formulated fot cannabis.
Second, run your lights 24/7. Cannabis is in a class of plants that will grow all the time it has light. An 18/6 light schedule encourages stretch and lengthens veg times.
I believe that you are going to find it extremely difficult to convert a refrigerator to a grow space and I think you will also find it too small. One of the things with LED s is that they need to be kept 18-24" from the canopy, which means you need some height to your space. Have you considered a tent?
Unfortunately I have been quite unhappy with my Mars LEDs. First, keepin mind that the LEDs only run at about 1/2 power, so that your 300w is actually less than150w. You might want to switch to the 600w now, but keep vegging if it is still small.
Why can't you post a pic?