in P8 SR0, tone gain is the unlabelled slider in dimension3d's (partners) render firefly script, next to the tone-mapping "enable" checkbox.  in att img, it's slider with value 2.34.  it's my hope that one of the previous or succeeding msgs in this thread may discuss the optical theory behind tone-mapping and the apparent empirical value of the gain variable in getting better render results, once SR1 is in widespread use.