For Fixed Assets , I have done up 2 custom functions for handling depreciation calculations relating to Straight line, Double Declining & Sum of the years digits.
DEPRE ~ for arriving at the respective yearly depreciation expense
ACCDEPRE ~ for arriving at the Accumulated Depreciation amount as at a specified period
For Clarity, an example worksheet is as attached below for illustrating on how these 2 functions roughly work. Hope it helps
Download Example file