Calculating depreciation amount after subtracting the opening accumulated depreciation?

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