I think your understanding of the “power” of the brown scapular is incorrect. I downloaded some information from carmelnet.org that might help, but I don’t profess to be either a Carmelite nor knowledgable in details of the scapular.
The Carmelite Scapular is not:
a magical charm to protect you
an automatic guarantee of salvation
an excuse for not living up to the demands of the Christian life
It is a sign:
which has been approved by the Church for over seven centuries;
which stands for the decision to
follow Jesus like Mary:
be open to God and to his will
be guided by faith, hope, and love
to pray at all times
to discover God present in all that happens around us.
Like any sacramental, it is a private devotion. My understanding is that whatever promise was attached to the brown scapular was directed to Carmelites, not laity who might wear it:
Also from Carmelnet.org:
Originally the scapular and any spiritual privileges attached to it were retained exclusively by the Carmelite friars. According to Richard Copsey, laity who were affiliated to the Carmelite Order in some way wore, as a sign of their affiliation, not the scapular but a white mantle. The mantle had originally been more important for the friars’ identity than the scapular. The late thirteenth and early fourteenth century constitutions refer to the mantle, not the scapular, as the signum suae religionis et professionis, the public identification of the order.  The friars needed to be told to wear their scapulars. Consequently in the first century or so of the Order’s history, it was the mantle, not the scapular, that was used to show the affiliation of laity to the Carmelite Order. This changed as the story of the scapular vision began to spread.
It is important to note that in the earliest accounts of the vision, the promise is made not to whoever wears the scapular, but to any Carmelite wearing the scapular. Given this, as the story became popular and spread, there was a clamor on the part of many devout people to have a share in this promise. The restriction of the scapular to the friars (and presumably after 1452 to the nuns) began to change at some point in the late fifteenth century. An important step in this change was the evolution of the Scapular Confraternity which provided the loose affiliation to the Order that gave people permission to wear the scapular without having all the demands of tertiaries. The small scapular with which we are familiar and which can be worn beneath secular clothing became a very popular sacramental as large numbers of laity were enrolled in the Scapular Confraternity.
The site seems to have quite a bit of information on the Carmelites in general and the scapular in particular. I hope the information helps you with your question.