Dunning Issue in ECC 6.0

Hi Experts,

I am having some issues in dunning, looking forward some help on this. Here is the requirement. The company sends out dunning letter every 15th and 20th of the month for the items due for last month, for example if a customer has transaction from Feb 1 to 28, he is due in making the payment before March 15, else he would get the first dunning letter on March 15th, if he still fails another letter will be sent on 20th of March with interest of 2%. If the customer still fails to make payment, another dunning notice will be sent on April 15th and if still he fails to make payment another notice will be sent on april 20th with interest. I tried many possibilities, either it works for 15 or for 20th of the month, it doesnt work for both together. If the customer is due by march15th, be might still make transaction between march 1 thru march 15, which of these items shouldnt come in dunning letter, now here is the problem im facing, these items come in dunning notice as dunning level 0.

I know i am missing something and confused on each field. Could somebody let me know what needs to setup for

Dunning interval in days

Min. days in arrears

Line item grace periods

And in dunning level

Days in arrears: (for all dunning levels)

Calculat interest

Always dun

print all items

payment deadline

Thanks in advance

Best Regards

Jen