Yeah I actually labbed them last night. Midscreen you can get b2(KB), f2, d12, d12xxdbf1 or b2(KB), f2, 12, d12xxdbf1. I kinda gave up on the b2 after that because that is for sure the optimal unless you can get something with standing 4 or something silly. You can also do 1(AA), 1, 1, 12, d12 with the first standing 1 being an anti-air. I don't think I'm going to be incorporating f2 immediately because it needs to be really solid to pull that one off but the other ones aren't bad at all. Just takes a little practice because we aren't used to extending combos so much with Jax...typically don't need to. Basically, you can get 1, 1, 12, d12 or 1, 1, d12, d12 after any launcher midscreen. After a f2 AA you need to do 1, 12, d12. That is really tight though, especially in the heat of a match so I tend to lean on f2(AA), 12, d12.
It all depends on the KB combo you are trying to get too. For a command grab KB, these work just fine; you only need to cancel the d12 into command grab and get well above 40% for an AA or well above 50% for a b2 KB. For the heated arms KB, you need to end in 12. You may need to even shorten the combo depending on height and all that good stuff. The most consistent I found was the one everyone knows - starter, 1, 12.
Corner, I don't have a clue. For some other good stuff, air-to-air jk leads to KB if you are ready for it. j3(air-to-air), f2xxdbf1 for over 40%. You can also get a KB off of a high height d2 with d2, d12xxdbf1 for over 40%. I believe these are over 40% but they might be over 50%. I don't really remember these situational ones exactly. You can get the same AA punishes with d3 as well. So d3(AA), 1, 12, d12xxdbf1.
I feel like I'm missing something that I found but that is for sure the majority of it all. Some/most is probably known, but I thought I would list it all for completeness.
Edit: If you want to land the bf34 KB, you should use b2(KB), 1, 12xxbf34. If you want to land the command grab KB, you should use b2(KB), f2, d12, d12xxdbf1. I probably should have stressed that. And there are micro dashes in between everything for the most part. I like to mash forward while I'm doing all of this stuff so it's less likely to drop.