I think the information is really useful for the OP, and for all of us.
You said you've chosen a lighter arrow because it's more forgiving when yardage is misjudged. But am I correct that you felt like the difference in the scenario you described is 8", and not something less than 3"?
I think folks generally look at the calculation like you did, and see such a drastic drop off in POI, all other things being equal. And they assume that the trajectory deterioration of a heavier arrow is too much to live with.
It pays to take a few minutes and run the trap of comparing the arrow weight/trajectories in real world scenarios. I think most people will find that they can shoot a significantly heavier arrow than they had imagined. This boils down to just how efficient compound bows have become, on top of the fact that most of us can guess within 10 yards of actual distance without ranging. This means you're adjusting your sight, or choosing a pin, that is close to the actual distance. As soon as you do that, much of the difference in trajectory between heavy and light arrows vanishes.
In my examples above, I'm not saying it's good to be 16" low when you thought he was standing at 50, and he's actually standing at 60! What I am saying, is that being 13" low in that same scenario is just as terrible!
Another way to look at it, that may help some folks:
If we want to assign a margin of error limit of +-4" (an 8" kill zone essentially), with our zero set at 50, that would mean that with a 550gr arrow (sticking with 70/30 or your speeds of 266), as long as the elk is 46.5-53 yards, he's dead. This means I can be off by +3.5, -3 yards - taking the lower of the two to be conservative, my total span the elk can be standing in, and I kill him, is 18'
The 450gr (290fps) arrow would allow a higher margin of error, with the range widening to 45.5-53.5 yards. I can be off +4.5, -3.5 yards. Total span 21'
You see - there is almost no difference in real world trajectory in these two scenarios, assuming all inputs are constant minus arrow weight. Gaining 1 yard would represent a 2% increase in margin of error at 50 yards.
Now, is the 2% in margin of error you gain, worth the 19% reduction in arrow mass? That is ultimately up to the shooter.
The difference between 240fps and 300fps represents a 6% increase in margin for error. Arrow weights for that speed gap with your bow? 650gr versus 385gr.
Bottom line, you are correct that lighter, faster arrows allow more room for error in judging distances. But how much it actually matters in hunting scenarios is vastly overvalued by all of us.
