Help with depreciation

Hi I am Canadian and the depreciation works as follow :

Say I bought photographic equipment at 1000$, it depreciated at 20% per year. But for the first year you can only depreciated 20% of half the value of the item (so 20% of 500$ = 100$).

Do you know how could I automate this?

I tried Manual entry + written down value, but it doesn’t seem to work

Thank you for your help!