Potatoes - how deep?

       
I emptied out my potato trial buckets today, very disappointed in the results from both the paper and the soil, the neglected trial in the greenhouse gave a better harvest, the difference being that I added chicken pellets to the soil of the first trial and  non to the second but added comfrey or seaweed to the water once a week.
So as I have now got two empty buckets, lots of newspapers and some seed potatoes I will go again, with chicken pellets in the base soil, shredded newspaper and applying a seaweed and comfrey feed on alternative weeks to one bucket, the other will just get water and hopefully will stone good clean potatoes for Christmas