Hello Paul,
Thank you for your message.
At present the way it is set up is to apply a percentage across all sales. So for example if 80% of your sales were ID applicable you enter this as your percentage. You then enter the percentage of that amount that will be advanced from the Invoice Discounting company.
I am working on an upgraded version which addresses this, but this may take some time to finalise.
At present the new version will probably be able to cope with different ID advance rates too.
Sorry this may not be the answer you were looking for.
If you have any further questions, please let me know.