The Rebbe responds to a concern about the correct positioning of tefillin shel rosh, advising the established practice of asking someone nearby to check, as is relied upon in many areas of Torah. He expresses confidence that the writer maintains the three daily study portions of Chumash, Tehillim, and Tanya, concluding with a blessing for success.
https://www.torahrecordings.com/rebbe/igroskodesh/014/005/5137