Check out this calculator for some assistance:
I input 0 mg/l initial chlorine and 1mg/L desired residual...which is false because you theoretically would end up at zero residual as the available chlorine will be used in the reaction to precipitate the iron. But if we do in fact use the 1:1 ratio of hypo to iron, then we can use your current iron concentration in the "desired" box. Although you stated an iron value of 0.81, I used 1.0 instead to account for some slight upward variance in your iron level. This should then leave little chlorine residual leftover. In fact, most DPD chlorine test kits would struggle to accurately measure down to that last 0.2 mg/l of residual on a repeated and accurate basis. ORP might be a better bet. But what you'd ideally need to do is measure the iron after the dosing which requires a spectrophotometer and the associated reagents. If you can get an inside track with your local area water plant, they might do that test for you.
So anyway, I set the output for mL and that value ends up being 0.13144 mL. How much is that? Essentially 2 drops from your run of the mill eyedropper.
Dang, that link isn't going to work so instead go here:
And then scroll all the way down to the bottom and click on the link titled "calculator for water disinfection using 12.5% sodium hypochlorite solution". Then you'll see what I mean about the input variables I mentioned above.