This is the part that both Dewey and LC use as their only major divider. Dewey made some attempt to keep a continuum in his classification, LC pretty much ignored it. Method of study or use. Oceanography as a subject of study was split between several libraries on my campus. In reflecting on the books, I decided that method of study was as much an identifier as the subject being studied.
Mathematical physics has much in common with mathematical ecology, for instance.
Arranged as level, method, subject, it should be relatively easy to code 'advanced philosophy of science', 'introductory mathematical physics', 'intermediate physical oceanography', and the like. These are less generally applicable, so there might be some specialization of the 5 reserved spaces for some of the major codes. Something like this does preserve the scheme for US geography books mentioned above. So 0A1.13456.A31 would be something like Asimov's book on the history of mathematics in the middle east during 700-1200. I don't wind up with a shorter coding that LC, and typically am longer than Dewey, but that wasn't my complaint about either system.