Quite frankly 1.9 is absolutely irrelevant to the problem you're bringing up.
Randomly contributing guarantees you'll have a conflict of interest between your goals and everyone else. They could have been doing swaps, exchanges, self-leveling or something else entirely. Result would have been the exact same: competing with everyone that intends to contribute.
If you're at all concerned about what the Owner plans for their building, then don't ever randomly contribute. Talk to them instead. Get a deal. You might find yourself bumped off by a random contributor, but you'll have a significantly reduced chance of this occurring if you talk to the Owner first to get a agreed price.
The main reason making a deal beforehand reduces getting bumped is because Great Building Owners tend to deliberately go out of their way to bump off random contributors even if the random contributed the amount the Owner intended to get
Regarding your idea: My main concern is it could prevent players from ever using a better method, or even just trying to improve the methods we have now. Some players use rates higher than 1.9. At one point it used to be 1.8 as the standard. Prior to that Swaps were the standard. If you put a 1.9 only indicator and stop there..... pretty good chance every method that would have been a improvement to 1.9 would automatically get rejected by the playerbase
Now if you put a interface that's customizable with absolutely anything you could possibly come up with, then it could be used for what you want while being flexible for if someone comes up with a different method. The contributor interface could certainly be improved to make calculations easier, but that's true for every method not just 1.9