Depreciation Method

Hi Guys,

I am trying to create  new depreciation method.

Our Tax Calendar has got 12 periods how ever when it comes to plant and machinery the new rule for tax should be 40% to be depreciated in the 1st month of the 1st year and the balance at 20% in the first month of the following 3 years.

Therefore

Y1 (Period 1) - 40% (Balance of the year no depreciation)

Y2 (Period 1) - 20% (Balance of the year no depreciation)

Y3 (Period 1) - 20% (Balance of the year no depreciation)

Y4 (Period 1) - 20% (Balance of the year no depreciation)

What would be the best way to se this depreciation method, or how would you do this?