There is no need to worry about it. Yes, 1 part washing soda(baked baking soda) to 4 or 5 parts baking soda is the ratio and about 1/2 cup of it will dissolve in a gallon of RO water, but please let me help you be sensible about this.
The truth is, Baking Soda by itself is just fine. The only reason some people use a mixture of the two is because of the tiny temporary drop in pH that occurs with baking soda alone. But honestly, the temporary drop is nothing to be concerned about. We are always stressing here on the MB that when there is good flow and gas exchange at the water surface the pH is fine and does not even need to be checked. We also talk about pH being better when Alk is good. So even when using baking soda alone, that is still the case. The tank won't even notice. The slight drop in pH only lasts for an hour and ends up being improved anyway because Alkalinity is being increased.
Please don't worry about the amount of baking soda to dissolve in RO water. Just add 1/4 cup and then a little more until you see that it's saturated and some undissolved Baking soda is on the bottom. Whatever is left will dissolve later when some of the solution is poured into the tank and the container is refilled with RO water. No harm, nothing to worry about.
I hope this makes sense and puts you more at ease.