Matt I don't think that will work. You need to use visual basic to update a field on another table not related to the form otherwise you get a #Name# result. The only other way I can think of is by including the second table as a subform (note must be linked by common field with the main table to work)within the main form and using a set value macro if you do not want to use vb. If you decide on this route create a text box in the subform footer and get the value in there first. Then work on the transfer to the correct line.
Alternatively you could use an update query after you are finished with the form to perform this task.