Would not the percentage of cooling offered by the board be relative to the cooling effect being applied to the CPU?
i.e. say the board has a C/W of 1.5.
We stick on a heatsink with a C/W of 0.5.
The total system C/W then becomes (1.5 x 0.5) / (1.5 +0.5) = 0.75 / 2 = 0.375. The motherboard now plays a 25% role in the cooling effect.
If we now stick on a waterblock with a C/W of 0.2, we have a total C/W of (1.5 x 0.2) / (1.5 + 0.2) = 0.176, and the motherboard now plays a ~12% role in the total cooling effect.
Still a significant amount of cooling, sure, but a fixed figure of 20% surely cannot be relied upon.
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