Calculating arrears should be simple so, NO there is no great arrears formula.
Arrears = difference between what the loan balance was expected to be according to the credit agreement and what it actually is!
Can't understand how
"...the arrears are increasing with every transaction..." if you are sticking to the terms in the credit agreement.
If you missed one payment but paid all others, in full and on time, then you would always be one payment in arrears.
Only way arrears continually increase is if you continually pay less than agreed - that would include paying the correct amount but paying it late.
If you paid each repayment late and/or short then the arrears would creep up.
Two simple examples below
Borrow €100 on 1st Jan 2013 to be repaid in 10 x weekly repayments of €10.
(Schedule = expected loan balance as per agreement)
This has one missed payment and arrears remain at 1 payment/week in arrears:
Date | Balance | Repaid |Schedule| Arrears
01/01/2013|100||100|0
08/01/2013|90|10|90|0
15/01/2013|90||80|10
22/01/2013|80|10|70|10
29/01/2013|70|10|60|10
05/02/2013|60|10|50|10
12/02/2013|50|10|40|10
16/02/2013|40|10|30|10
26/02/2013|30|10|20|10
05/03/2013|20|10|10|10
12/03/2013|10|10|0|10
19/03/2013|0|10|0|0
This one has continious short payments so arrears creep upwards
Date | Balance | Repaid |Schedule| Arrears
01/01/2013|100|0|100|0
08/01/2013|91|9|90|1
15/01/2013|82|9|80|2
22/01/2013|73|9|70|3
29/01/2013|64|9|60|4
05/02/2013|55|9|50|5
12/02/2013|46|9|40|6
16/02/2013|37|9|30|7
26/02/2013|28|9|20|8
05/03/2013|19|9|10|9
12/03/2013|10|9|0|10
19/03/2013|1|9|0|1
26/03/2013|0|1|0|0
HTH
and the arrears are increasing with every transaction.