12" = 12 inches = 30(.5) cm
I would guess about 5~8cm of manure/compost in the bottom, 3~5cm of soil buffer... I'll let you do the rest of the math!
Remember, there's less air movement in the hole so if your area is humid
you might have problem with fungal diseases. If you have clay soil that doesn't drain well, you might end up with a "sunken bucket" filled with water and potatoes will rot.
Just for the record, I prefer the sheet mulch/lasagna garden method in which you put the potatoes near the bottom layer before piling up the straw/compost/soil/mulch. In my area where humidity can be high even in the middle of a drought, I don't dig that deep in established beds -- maybe about 9" down.... I like to end up with a saucer shaped depression to collect water, and when I hill up, I try to keep a slight depression around the stems and make a moat surrounding the hill for the same purpose.