I can't think of another Automotive Diagnostic Tool or gauge that requires total accuracy than does the Timing Light.
All the other tools and gauges we use are "relative" to itself. Even the venerable "Feeler Gauge" is subject to "Feel" of the individual. Of course you're probably talking about only a couple of hundred thousanths, plus or minus, but nothing requires the precision of the timing light. And you have to assume it's right everytime you use it . .
Like I said earlier, it doesn't hurt to compare it to somebody else's every so often and to send it back to the manufacturer for claibration once in a while . .