There's too many variables Bob. However a 100g/litre of each diluted 1+10 works very fast, that's 10g/litre, your talking a touch under 3g/litre, I'd start by diluting that a further 1+9 (1:10) so you have about 3g/litre Ferricyanide & KBr

One nice point about using this bleach is if it goes too far, you just wash and re-develop, the room light is usually sufficient to re-expose the bleached emulsion.

Ian