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Without doing a look-up, as best as I remember is to make the rice powder about 10-20% of the mortar/concrete mix. Don't count the sand in that %.
Its more of a "this looks about right" thing for me. Just when it looks like its the consistency that will work for what you're using it for. There might be some specific recipe on the internet. Probably is.
Over here, Taiwan, bags of rice powder are sold at the 'Home Depot, Lowes' type sores ready to use.
They also have it pre-mixed for use...but it costs more.

You can make it your self by just grinding up rice to a fine powder. If you do that, use it quickly because it is very moisture absorbent and you will have a solid rock if you're not careful.