How to auto populate a bound control within subform with sequential numbers.

Hello,

I created a database to record laboratory freezer inventory. The freezer is divided by shelves. On each shelf is a rack, which holds boxes. Each box contains 81 different specimens. In this database there is a form titled, "Test," that is bound to a table titled, "Molecular Freezer Inventory Mapping." Within the main form is a subform titled, "TestSubform," that is bound to a table titled, "PHIMS/OpenELIS Clinical Specimen Numbers." The Molecular Freezer Inventory Mapping table is mainly used to document: BoxID, Year, Analyte, SpecimenMatrix, Freezer, Shelf, Rack and Box. The PHIMS/OpenELIS Clinical Specimen Numbers table documents: PHIMS/OpenELISNo, MatrixVolume, BoxPosition and BoxID. BoxID is used to link these two tables together.

Here is my problem...
My form is working correctly; however, I would like to have Access automatically populate the BoxPosition number for me upon creation of a new record within the subform. I found a bit of code, provided by Stephen Lebans, but I don't know how to alter it for bound controls. Any help would be greatly appreciated! I have attached a version of my database for reference.

Thank you very much!
Test-Molecular-Inventory-Copy83115.accdb