I'm pretty sure that ground level is at y = 64, and that bedrock is at y = 0 (not as sure about the last one though), which would mean (60 × 60 × 64) ÷ (54 × 64) = 66.67 DCs.
*Everyone retreats to the frontier and slowly but surely everyone goes insane from lack of Frappuccinos*
This gonna cause for one serious *bang...* It's gonna be so loud it's gonna come through out of your speakers at home so loud that it might shake your house.
The kind of guy who will double-post to quote himself in a reply. Truly, you are a dangerous man on the edge, and worthy of my fear.