Method to calculate depreciation amount after subtracting the opening accumulated depreciation?

Good morning to every one,
what is the method to calculate the depreciation amount(every month), after subtracting the amount of openings accumulated depreciation from gross purchase amount? in erpnext system
for eg: 20000 (gross purchase amount
5785.64(Opening accumulated depreciation amount already depreciated)
total number of depreciations are 169 than depreciation amount for every month is 84 how it’s calculating in the system?