I dont think I agree with your rule of thumb that an arrowhead should be equal in grains to the draw weight in pounds, nor that the head should be light so that it does not affect the flight of the arrow. If the second part were true, then arrowheads of very light weight would produce the best arrow flight. Instead , we find that good flight is a combination of static spine, length, tip weight and shaft weight. No matter the material, an arrowhead is the right weight when , combined with these other factors, we get good flight. It seems to me that to get an arrow with a 50 grain tip to fly well, we would need either a very long or very weak arrow. But most of us shoot arrows that are about 30 inches long, and I have never tried it, but to get a 30 inch arrow with a 50 grain tip to fly well out of a "regular" selfbow, we'd need a very weak spine.
When I make stone tipped arrows, I try to make them as thick as I can, about 11/32nd. I also sometimes add weight in the shaft or hafting materials. Even with these efforts, I feel that my arrows still fly like a stiff arrow (tail right, and impacting left of where I expect. This is one reason I have switched to steel points and weighted adapters for hunting.
Now I realise that in the old days , millions of animals were killed with light points. But I still believe that a combination of factors will contribute to good arrow flight, and that there is no "one size fits all" rule for point weight.
Patric i am always willing to change my opinion and discuss things. I would love tohear more of what you have to say on this. Your opinion is highly valued here.J