Chakram’s have a base of 60 DMG as seen in the wiki. You want to calculate the DMG of a lv 20 Chakram. Putting those 2 values into the formula you get:
(60)(1+20/4)(max(1,20/20))
(60)(1+20/4)(max(1,1)) -> since the numbers in the max function is the same the value returned is 1
(60)(1+20/4)(1)
(60)(1+5)
(60)(6)
360