The wikipedia article has some great info. on the historical progression of this question.
Using <PERSON>'s equations, one recovers the wave equation for electric and magnetic fields. From these equations, <PERSON> postulated that light can be thought of as an electromagnetic wave since the electric and magnetic fields solve the wave equation with a phase velocity of $c$. This theoretical value of the phase velocity of light waves is exactly $299,792,458 \frac{m}{s}$.
I mean is it 100 percent accurate?
Nothing is ever 100% accurate - that's an idealization of the human mind since there is always systematic errors in the measurement process (even if perfect humans are conducting the experiments). 840
So, no, it is not 100% accurate. It's worth noting that there's a difference between accuracy and precision.
As the wiki article shows, the precision and the accuracy of the measurements of the speed of light have improved greatly with time: as better measurement techniques are used, the measurements agree with each other better and better, and any one of the measurements agree with the theoretical value of $c$ better and better.
So, although we cannot have 100% accuracy in principle, we can have such small uncertainties in the measurement that we might as well consider the value to be 100% accurate. From the wiki article,
After centuries of increasingly precise measurements, in 1975 the speed of light was known to be $299792458 m/s$ with a measurement uncertainty of 4 parts per billion.
A measurement uncertainty of 4 parts per billion is very small, i.e. 0.0000001% of uncertainty: it's like saying for every million years you live you only fudge 30 seconds of it.
Lastly, in 1983, the meter was redefined in the International System of Units (SI) as the distance traveled by light in vacuum in 1/299792458 of a second. This is justified because, as I said above, the precision of measurements of $c$ are so precise that we might as well just define $c$ to be exactly the value that they all (within certainty) agree upon - this value as it turns out is also what <PERSON>'s equations predict.
Thus, yes the value of $c$ is defined to be exactly that given by <PERSON>'s equations, but importantly this is justified because the measurements are very very very precise. 840