Can the TotalLength be based on a #define variable like desc_fs_configuration_length.
Then desc_fs_configuration_length could be based on further defines like:
#define desc_cdc_length xx
#define desc_midi1_length yy
#define desc_midi2_length zz
#define desc_fs_configuration_length desc_cdc_length + desc_midi1_length + desc_midi2_length