I have the same strategy if I'm struggling with a particular exercise: ultra-short, upbeat sessions, at least once a day, preferably two or three times a day. The only thing that bothered me a little was that you told Finch the first time, when she cut the corner, was not good enough, but I wondered how she knew what exactly wasn't good enough. She stayed on stay, went on jump, cleared the jump and came back to you. How does she know what her mistake was? Since she didn't cut the corner the second time, maybe she's figuring it out by trial and error.
I use a pad of artificial turf to define the "no-landing" zone. I wouldn't want the no-landing pad to be something that might hurt their paws and make them worried about jumping. I want it to be something that feels different than the grass.
A video of a couple of jumps with my experienced dog, Pinyon. I rarely work on the broad jump with him anymore, but when I do, I always put the no-landing pad down in case he starts "drifting". I also like to do the jump with me standing close to him to encourage him to look forward and not at me. With Pinyon, I do a mostly formal version, the way I'd do it in the ring: giving the BJ pre-cue (in my case, saying "broad jump" and doing a slight crouch and point) and speaking the judge's commands and the responses I'd give in the ring.
A video of Hawthorn, who hasn't been shown in Open yet. He is 2 years old. Sorry, this is a too-long video, but I wanted to show the sequence of increasing level of difficulty. We haven't worked on the BJ for a couple of months, so I expected he would touch the "no-landing" zone. To my surprise, he did not. Since he is still learning, I started by putting a cone on the no-landing pad and pointing to it and saying "leave-it" to remind him it's there. We did three reps with the cone, each with me moving closer and closer to him. Then, I took the cone away and did three more reps, each harder. With him, he gets lured into the setup position and rewarded for the setup to keep his attitude up and to avoid having to mix in lessons about setting up.