Dear FLUKA experts:
Using the FLUKA code, I am calculating the dose absorbed by a detector made of A-150 plastic, shaped as a cube with a side length of 19.19 cm and a volume of 7067 cm³. For this, I am using the USRBIN command with WHAT(2) = DOSE (ID - 228). Is it necessary to divide the data obtained from the USRBIN command by the volume of the detector to get the correct result?

It depends on the USRBIN type (WHAT(1)).
If you scored by region, you must divide the resulting value by the region volume in cm^3 (this way getting the average dose over the detector, in GeV/g).
If you adopted a Cartesian or cylindrical scoring mesh, the resulting values are already in GeV/g, since the mesh bin volume is known to FLUKA, which used it.