How does the area of marble of stair step platform calculate? Is 2cm of marble overhanging platform area? If granite is not made at the skirting board (not the skirting line) of the stair step, and the step width is 28cm, how much material should be provided? Please give me more advice

How does the area of marble of stair step platform calculate? Is 2cm of marble overhanging platform area? If granite is not made at the skirting board (not the skirting line) of the stair step, and the step width is 28cm, how much material should be provided? Please give me more advice

It's just the same