Color values in Sun rasterfiles are eight bits wide, so pnmtorast will automatically scale colors to have a maxval of 255. An extra pnmdepth step is not necessary.
The -standard flag forces the result to be in RT_STANDARD form; the -rle flag, RT_BYTE_ENCODED, which is smaller but, well, less standard. The default is -rle.
All flags can be abbreviated to their shortest unique prefix.