Best approach to a defaulted field based on the user

I suggest a table of Users where the the key field is User.  In that field you would list their Admin as well in another field.

Then make a relationship where 1 employee list entry has many detail records, and lookup the Admin userid.

Then either you can default the Employee field on your detail records to the current user or you can use a form rule to populate it to the current user if its blank.