Just mix it at a 6:1 ratio. That's 857 gm. of baking soda to 143 gm. of washing soda to make a 1 kg. batch. I've tested it against the Kent super buffer stuff and noticed no difference at all except that my wallet is a lot heavier afterwards. I think the commercial stuff has a pich of borate in it. I suppose you could get some boric acid and mix it in with a little extra washing soda to neutralize it but I have never bothered with it. My corals never noticed the difference. Boric acid is pretty easy to find. I use it to make a DIY ant bait.

Gary