Bronica AE II E Finder Owners Manual
Since I never use a TTL flash with my Bronica's, I'll give you my suggestion for a great solution. Using your TTL Metz flash, (make sure you've connected the TTL cord for Bronica ETRsi) take 8 shots at different apertures on A (Automatic), then take 8 shots at different apertures on M (Manual). I realize this will waste one 120 roll of film, but one negative will stand out as your perfect exposure. Yes, whenever you reduce the ISO in half, your basically overexposing the film by one f/stop. Just remember that this overexposes everything your shooting from foreground to background. With flash, this may cause your highlights to wash out, but any good lab could print for the highlights and make the background go darker. I used to shoot Vericolor at ISO 125 instead of it's rated ISO of 160.