Configuration of new depreciation key

Hi Experts

I am working on configuring depreciation key for the first time and need your guidance.

Requirement of new dep key is as below.

a) depreciation for 1st year - 60% of the value

b) Depreciation for second year and so on - 20% each year on straight line method

Example : Cost: HKD 100,000

Depreciation for first year:

HKD 60,000 (Cost HKD100,000 X 60%)

Depreciation for second year and so forth:

HKD 8,000 (Cost HKD40,000 X 20%)

Can you please suggest how this depreciation key can be created or any provide any document to understand this.

Regards,

Santosh