What's a good ratio of BRS soda ash and bicarbonate?

[Cambria]

So before when I was dosing my 42 gallon, I used Kent Superbuffer-dKH which has both carbonate and bicarbonate. Now that I've transferred my sps to a 125 gallon tank, I started using BRS soda ash (because the Kent was being used up like crazy and it costs too much). But my birdsnest, montis, and a couple of my acros started to die because I think it was having pH swings. I stopped dosing and just did some water changes and everything is coming back slowly and looks good.

I dose manually. (I can't get a doser right now and I don't have a pH test). So my question is what is a good ratio of the soda ash and bicarbonate? I read somewhere that 85% bicarbonate and 15% soda ash is good because it doesn't affect pH at all. And I wanted something close to the Kent I was using.

[bertoni]

The 85:15 ratio is fine for a mixture that will leave the pH unchanged. In reality, using all bicarbonate is fine, as the pH change will be very small and temporary. The ratio is an approximate ratio by volume, and that's more than good enough for a formula. A total of something like 1⅛ cups will dissolve per gallon of water.