Opacity of a density of 2.6 is 10-2.6 = 0.002511
Exposure necessary at film plane is 0.8/ISO so 0.8/320 = 0.0025 lux s
Exposure metered above just above the Stouffer needs to be 0.0025/0.002511 = 1.13 lux s
Looking at the earlier given formula to convert a light meter f/stop reading to lux s:
Number-of-stops-to-right-of-f/1.0 = log2 (exp/2.5)
so, log2 1.13/2.5 = -1.15
Are you sure this last step is correct? As far as I understood, the value 'exp' in your equation should be an illuminance value [lux] instead of an exposure [lux s].