Maybe this might help you...I've been down this path and it took me a lot of time/effort/aggravation to get it right. I had the same issue where I couldn't quite get the throw correct and if I did, the throttle would stick a little bit open. You may be working on a big-block so I'm not sure how this translates but I see from your pic that you are using the same throttle spring/cable bracket from Summit that I am. So I'll share what I did and maybe it will help.
For my 408 in my 72 Demon, with an 850 DP Holley and a Super Victor intake manifold, the carb sits up pretty tall. So a couple spare cables I had would not work. Nor would the original stock cable from my 71 Demon 340. Arrgghhhh...what a stupid problem to have to work on..... But of course without a properly functioning throttle cable, you don't have much to crow about!
So I took a bunch of measurements of what I had and talked to a guy at Pioneer auto parts (their cables are sold through Summit) and he was kind enough to get me a few measurements from some of their cables. So I ordered 4 of them from Summit in hopes I could make one of them work. Here is a little picture/table of the measurements of the cables (DCM is just my initials in the table). As noted, I ended up using #8420 and it allows for full throttle travel without any throttle hang-up at idle. The one I used has the "square" end where the plastic end pushes through the pedal and the legs "pop open" to hold it there. I prefer the "round" style with the clip that holds them in but I didn't end up with any that would work. Since the #8420 does not have the flare to hold it in the bracket, I had to use the set screw that is on the back side to hold it there.
Hope this helps - good luck!!
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Now if I can just address the fact that the return spring holder arm hits the bottom of the 5" air filter base.....