As this is Meticulous Maynard’s Chain Guide, I would suggest a new topic 
There I would then ask on what side of your drawing the sled is. Chain jumps happened for me when the sled was moving to the top of the sheet, on the side with the shortest chain to the sled. This side has the longest loose chain and the rubber bands can’t compensate for the chains trying to go vertical. Because the frame and the motors are tilted, the loose chain is coming in from a more vertical angle and when you feed that chain out, the side of the chain will try to ride on the tip of the tooth of the sprocket. With a noticeable ‘Bang’ the chain will either skip a link or fall into its meant position. If you have chain jumps starting from the sled side of the chain, your sprockets and sled-chain-mounting-points are not parallel to the work piece. With the pantographs you add or remove mounting spacers, with the brackets you go one hole up or down. One more reason for chain jumps could also be flexing top beams, as both sprockets get turned out of alignment. I apologize if you knew already, hope it still it can help someone else.
The changing pivot point of the chain on the sprocket depending on the chain angle, is an error that is already, or can be compensated by the soft- or firmware, I think. I agree the a small idler can reduce the amount of error and would make room to experiment with different sprocket sizes.
Edit:
This is answered here: PDF of the formular