What would be logged as the destination for an emergency landing in a field?
I would just write "off fld" in the destination box, if there is one, and describe the location in Remarks.  "Emerg ldng on I-90 2 mi S of Whereverville"  or "Pracaut lndg field 2 mi W of Middleofnowheretown".

You aren't going to get audited over that kind of stuff so don't worry about hyper precision.  If anybody ever looks at your logbook it's just going to be to validate flying times for the purpose of a license or rating.  The other stuff is mostly for your own historical record.