Custom Function Help

Hi,

I need help with a custom function.

This is the scenerio i would like to happen.

I have a custom module called "Learning Consultants", these are my sales people. I record things like their target, actual sales, no. of leads and conversion rate.

At the moment, i manually add up the actual from any invoices produced in that month for that sales person.

What i would like to do is automatically add up the sub total of any invoice for that person in the current month and display it in the 'actual' field.

So when i create an invoice the sub total is added up in the corresponding invoice owners record in Learning Consultants module.

I've tried looking at deluge and while im ok working with scripts i just dont get it.

Chris