My rule of thumb is 9 to 10 grains of combined arrow/broadhead weight per pound of draw weight. I shoot a #55 recurve with a 495 grain arrow combo. My broadhead is a 3 blade 125 grain Razorcap and it goes clean throught them. It doesn't fly like a dart, it flies like a telephone pole launched from a catapult.
It's mass that does the killing, not speed.
And your comment about hitting the "sweet spot" but you couldn't find the deer even with a heavy blood trail.....Are you sure it was actually in the "sweet spot"? By that term I assume you are talking about the spot right behind the shoulder, which if you did in fact hit on a perfectly broadside shot presentation you should have found that deer within 100 to 200 yards max. A double lunged hit deer hit with a razor sharp arrow CANNOT GO much further than that. Once they loose a certain amount of blood they collapse..period. If they didn't, then you didn't hit where you thought you did, or your broadhead wasn't sharp, or if you were using one of those mechanical broadhead contraptions then it probably didn't open right, or open at all.
How far was the shot? What was the shot angle? (Broadside/quartering to/ quartering away) Were you in an elevated position? How long did you wait before going after the deer? What was the color and consistancy of the blood? What was the position of the deer's near side (the side closest to you)front shoulder when you shot? Define "sweet spot" in this situation.
That said, with that draw weight/arrow/ broadhead combo you were using, while being on the light side for my tastes, you STILL should have been able to kill those deer with no problem provided that you did hit them where you thought you did and you weren't taking 30+ yard shots.
Offer No Apologies.....
NRA Endowment Life Member