Have you tried setting the "value from last visited record" field option on this field?
Script trigger performed scripts could also save the most recently specified location to a global variable and then an auto-enter calculation could auto-enter the value of this global variable.
No I didn't try the first option, as the location may not be the last visited record.
A script trigger sounds like a much better option! I have a programming background but am new to FileMaker scripts, what's the best way to learn how to do this kind of thing? Thanks for your help!
what's the best way to learn how to do this kind of thing?
Try doing it. It should not be terribly difficult--especially if you've worked with event handlers in other systems such as Visual Basic for applications.
OnRecordLoad can perform a script that copies the location from the last portal row record to a global variable. OnObjectExit can be selected for the portal to perform a script to copy the value of the portal row being exited to this same variable. Setting up both should keep your variable set to the location most likely to be used for a new record in the portal.
Thanks a lot PhilModJunk! I think I have it all working now :)