Personally, I don't use a print for the residual HYPO test. You could use part of the white border if you wish. But for me it is just easier to cut a strip from a sheet of paper from the same batch and process it without exposure.
As I understand the residual test instructions, you remove the sheet from the wash, pat dry (it will be damp and emulsion thus soft) and place on the drop. Time for 2 minute then blot off. If the drop leaves a visible stain then wash some more. If you are testing on a finished print and the test says it passes, put the print back in the wash for 5 minutes or so to remove the test solution. It will over time leave a spot due to the silver nitrate in the test solution.
FYI : http://unblinkingeye.com/Articles/Ar.../archival.html is a good article to read.