Sooooo... I just pieced this together real quick..
It should theoretically allow you to achieve PERFECT hydration levels.. between 58% and 64%
r : weight of rice (g)
s : % moisture level of uncooked rice
p : % final hydration level, ideally between (0.58 and 0.64)
w : volume of additional water (g or ml)
(w+rs)/(r+w) = p
Hard part.. finding -s-
TO FIND the moisture level of uncooked rice..
1. Weigh your rice,
2. Heat it dry in a pan
3. Weigh it again
Do 2 to 3 trials.. you shouldn't need too much rice..
If anyone has any ideas to improve this.. please chime in..
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First of all, where did you find this formula?. And also I don't know much about rice.
Also all my working below assumes the following:
-s and p is relative to the weight of the rice i.e if s = 0.5 then a 1g grain of rice would be made of 0.5g of water
Anyways, why would anyone ever need to find 's'? If the point of that formula was to achieve the perfect hydration level 'p' then you wouldn't need to find 's' using that methodology as you would already know 'p' (which would be your desired hydration levels perfect for you, seeing as it ranges from 58%-64% so you'd be the one choosing this value as it's not a constant). Since you know 3/4 variables in that equation then you can just solve for 's' without having to weight it, heat it and weigh it again.
And lastly, from personal experience the the amount of water in uncooked rice (the ones you get from the store) is already low enough to begin with that it would practically be 0% . This means that s ≈ 0, close enough that we can treat it as 0 which means that 'rs' = 0 making the equation be (w)/(r + w) or (w/r) +1 = p which would make 'p' > 1 making it much larger than the perfect hydration values you stated. Doing a thought experiment, I were to buy uncooked rice dry it for a few days (such that s=0) and add no additional water to it I would somehow get 'p' with a value of 1 (0/r + 1 = p)
which doesn't add up since in this thought experiment I made the rice have no water in it at all and somehow it comes out as being totally made out of water.
TL;DR the finger rule is supreme :p