Being able to pin the spending on that line or a percent thereof should also be possible.
So if I set the spending at 10% above the line it should go up and down with the line. (OR 10% deficit spending for that matter)
It would be nice to get a notice if the "governor" that does that is unable to maintain the spending.
In regards to making it so that I would need to drag the pip twice to cross the line, I am not sure I like that idea. If I use CTRL to drag below the line and the pip stops on the line, I can then move the mouse right or left to find the exact percentage I want. If I then decide to drag again and see what something else looks like, I can keep holding the CTRL and stay there. But with your solution it will now drift below the the line.
We could still have it your way, but we would still need a button or CTRL to keep it up there in case we want to make adjustments.