If you use a water based coolant, like corsair's, EK cryofuel etc.. the additives are so minute you can consider it like distilled water for the calculation. they won't change much.
For DP ultra which is 20-35% glycol, the specific heat drops a bit, but you can still keep the same number if you like. those NTC sensors are not exactly precise, and are not calibrated so the whole calculation is more for shows, to get a rough idea of how many watts your rads take out.
For glycol i found this table :
I arbitrarily use 30% concentration at 30°C on mine, so the specific heat goes from 4185 for water, to 3828 for the glycol mix. basically negligible for our use.
The flow sensor is somewhat accurate, but the NTCs are crap, and we don't know the exact % of glycol, so we just stack margins of error to get a very symbolic value

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"good enuf"