Which ISU billing variant(s) to use

I am looking for advice on which billing variant(s) to use to subtract consumption from an annual block amount, which drives to a block rate. Annual consumption block is 10,000 units; price from 1-10,000 units is 20 cents/unit; from 10,000 – 99,999 units, 10 cents/unit.

Consumption in Month 1 is 3000 units at 20 cents/unit. (10,000 – 3,000 = 7,000 units left in block)

In month 2, consumption is 4000 units, again at 20 cents/unit. (7,000 – 4,000 = 3,000 units left)

In month 3, consumption is 4,000 units – 3,000 units at 20/cents; 1,000 units at 10 cents/unit.

In months 4 thru 12, all consumption at 10 cents/unit.

After month 12, reset consumption block to 10,000 units.