Certain formulas list a few ml of 1% Benzotriazole solution, like Crawley FX-37, it says 5 ml for the stock which is diluted 1:3-1:5, giving a total of 0.8-1.25 ml Benz. solution per litre working solution.
So I would aim 1-2 ml of a 1% Benz. solution per litre working solution.
I would add the Benz. solution when you prepare the working solution and not to the stock solution of HC 110.
We rather use 1% Benz.-solution when working with papers to change the tone towards cold or to "repair" fogged papers. We use 2-5 ml per litre of paper developer and adjust from there.