I have to tell you I would not do it that way, at all.
I would instead simply store the cost, useful_life, and when_acquired.
And then use a query that would calculate the accumulated depreciation on any given date by comparing the given date with when_acquired.
Your system is EXTREMELY vulnerable to error. If somebody runs the code to (example) add September's depreciation to all records on September 1st but then another person *ALSO* does that on September 3rd, then you would have accumulated two months of depreciation when you should have only accumulated one.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.