Any constant in physics or chemistry comes from experimental design and consistency in measurements. For example, if you divide multiple of pressure and volume with the multiple of moles and temperature, you will get the constant R for any ideal gas.
However, given how small planck’s constant is, how can we assume its accuracy?
In: Physics
Actually we know the exact value of the Planck constant nowadays, without any uncertainty.
That is because the Planck constant is one of the constants which is used to define the base units in the new SI system. So the kilogram is defined the way, that the plank value have this exact value. (Similar how it is with the meter and speed of light).
For how the value was determined historically other people has already explained
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