Attachment of Earning - not possible to set up % in a employee settings

In Iris PAYE Master for Attachment of Earning - Council Tax there was a column with amount of % available to set up based on Employee Net earning as per council guildance however in Staffology it is not possible. I have been advised that the software has tables in the background of the software, that will calculate the amount to be deducted based on the Total Amount to Pay and Previously Paid amounts entered which is incorrect as amount deducted was nearly double that IRIS was deducted on weekly basis therefore I had to manually change the amount to align with IRIS until migration process is completed.