Calculation Specific To Serial Number

I work for a graphic design firm and I've recently taken over the administration of our "Works in Progress" database, which includes project info, recording billable hours, invoicing, etc.

Up until now we've been invoicing for total hours spent, but now we would like to designate on the invoice whether the hours were 'administrative' or 'design'. Each project has a unique serial number, and when hours are entered under that serial they are designated either 'administrative' or 'design'.

I can do a 'find' and in our Time Log layout to get these numbers, but what i would like is to have 2 calculation fields on the Invoice layout that automatically calculate these hours as they are updated.

Any ideas? If you are intruigued, please let me know if I need to explain further. Thanks in advance!

Edited January 11, 200620 yr by Guest