I do not agree with inertia pulling the cam back advanced at all. Inertia does not overcome the valve frictional forces and why engines with variable cam timing have assist springs to pull the cam timing back from retard conditions as vs. no assist when retarding to begin with. If the inertia advances then the spring is unnecessary. The spring helps overcome the valve frictional forces. End effect is what you see with a motor mounted on a spintron or strobe unit timed to watch sprockets, the timing moves around both advance AND retard, it does not stay constant one way or the other. Why high output drag engines go to belt drives to dampen that out. If you pay attention to enough engines at rev and with wear, it becomes obvious the chains retard and stay that way even at full revs. You lose too much bottom end that shows back up when you replace chain. The old Boss 302 my friend and I had did it regular as clockwork, we put 3-4 chains in it. Also showed up on the Honda CBF bikes I drove, they laid down and died on bottom end when chain slacks and that chain as heavy as a car one, it showed pretty quick. 1/4 mile times slowed, and new chain they got faster even though with new chain you could short shift to better use the bottom end, with stretched chain you had to buzz it more to even get near the quicker times. The engines were actually cammed at around 100-105 LCs, when the chains stretch to 108 you were p-ssing in the wind, the engines dropped lots of power. Still seemed to rev fine up high but dead down lower and mid. The engines were peculiar because the 'new' trick 4 valve head didn't breathe at high rpm because valve package was too small. They would always rev to 10000 rpm but the midrange was what you shot for with cam timing, if retarded they still revved high easily but simply blew up while not making as much power. How most ended up in the yards with rods sticking out of side of motor, everybody though 'bike motor, high rpm' but no, they did NOT like it.
And, there is no standard at all that says aftermarket cams are ground with 5 degrees built into them. Some may but not all. The numbers are to be taken as the numbers and set up with retard in mind yourself. Doing otherwise would beg the question, 5 degrees in relation to WHAT?, the last thing someone dialing a cam exactly in needs. The numbers on cam cards are what you shoot for and build in any retard space yourself. There is no way they can control what sprocket you use and it could be off so better to just give straight up numbers to avoid customer confusion. In fact, I'll pose the question myself, or, 5 degrees in relation to WHAT? You begin to see the problem....... ........