Looks very serviceable and well done! The dart is real nice, could easily make meat. Looks like it is about 2 1/2 times as long as the atlatl. Might have to talk to you about some trade points like that. The finger loops would be a real good idea. You might try a thumb and forefinger grip on the dart when castling, or a dart rest. There are a number of sites that talk about dart speed, but I think it averages about 30% of what an arrow flies at. Bob Berg at thunderbird atlatl.com may have some info on that, I've not had access to a chronograph to test with. My friend, Ken Wee has a utube video, but I don't remember if it mentions speed of cast. Accuracy can be a challenge, I can usually keep the darts in the 4 x7 foot butts, almost all the time, but I need more practice to get a group!
Good work, didn't think of making darts with the extra long shoots!
Hawkdancer