My instinct would be to have the numbers not be visible to the user. You just have a rectangle with two colors, initially red on the right side and green on the left side. Clicking anywhere inside the rectangle changes the dividing line to be at that location. So clicking 90% of the way towards the right would make the left 90% be green and the right 10% be red. The backend would know that it corresponds to whatever number it corresponds to (+80 according to the scheme I gave earlier), but the user just has a qualitative feel for how much of the mass they’ve allocated to the good (green) color and how much to the bad (red) color.
My instinct would be to have the numbers not be visible to the user. You just have a rectangle with two colors, initially red on the right side and green on the left side. Clicking anywhere inside the rectangle changes the dividing line to be at that location. So clicking 90% of the way towards the right would make the left 90% be green and the right 10% be red. The backend would know that it corresponds to whatever number it corresponds to (+80 according to the scheme I gave earlier), but the user just has a qualitative feel for how much of the mass they’ve allocated to the good (green) color and how much to the bad (red) color.