Bug in Post Sourcing on transaction line items?

I've added an item column field on transaction forms that is used as part of the calculation called 'duration'. When the field has a value, the following client-script is performed: amount=(quantity*duration)*rate

Now if you happen to add one or more line items while the 'Customer:Job' field is empty and then enter a Customer:Job, the sourcing of various fields occurs and interestingly, so does a recalc of all of the line items setting them to: amount=qty*rate, effectively wiping out my calculation.

I tried putting an event handler in the post-sourcing event to go back through the line items and recalc them to use my field, but it seems that my function runs *before* this recalc so is a moot point.