I NEED SOME ASSISTANCE WITH THIS!!!!

I followed the DIY Tipster article on creating your own Redscale film i used a Fuji Reala 100 35mm, and from this result i have a feeling 100ISO film reversed may provide around a 20ISO possibly even lower? im going to attempt this again with a Kodak 200 35mm and do some long exposures. If anyonye can provide me with some assistance on aperture, exposure and ISO id be more than greatful my lightmeters dont do this method. FYI in this shot: 1/125 f2.8 50ISO (and its slight massively under exposed :s) was shot at dusk