You can fix a date for your khoms calculation. It could be the date you started your first job.
Let us assume it is 15.10.2020. During the fiscal year (15.10.2020 to 14.10.2021) you earned and spent money.
On 15.10.2021, just check what you have in hand and in account, and pay 20% of it.
The installments paid to settle the loan are exempted from khoms