Depending on your sourced information..you may find 5% as a safe cap,and others who say 10% is flyable ...note the 5 day rule in NJ...some courts may balk at language that allows LL to apply payments to fees first..thus setting state fortenant to always be late . I use a daily amount bit Im in PA not NJ..... In some places its important to,define late fees as added rent ...if you seek to collect rent

Id suggest you compare notes with larger local LLs ...ones likely to push as hard as experience supports .

One of my LL pals claims good results by allowing a discount for rents paid on or before due date .

Others have gone to mandatory bank auto payment plans with careful words via counsel.