AIST Report No 7

Give us the full equation and we might be able to comment.

as I understand, the stress is compute at 1 inch so the "1" is in fact, "1 inch"

Probably, as length unit seems to be inches. But hard to be sure without knowing what the figures refer to and where the force is acting.

t appears to be the thickness of something being bent, as stress = moment/Z, where Z = section modulus = I_t/(t/2). That would make (t/2 - 1) the moment arm for force P, the 1 being 1 in as you suggest. But why that should be the case, or what "the rated load applied 1 in from the outside edge...." means needs a diagram to confirm.

Cheers.......Codey