You shall find several variables.
The ***_rgs Variable tells the region that was marked. If you did not define any regions, there will be a "1" for each mark. Ideally, you have defined one region for each step of your ladder.
And then, there is the ***_pts variable. It has three comma-separated values for each mark. These are the X and Y coordinate, and the type of marker set.
So, if you take the second value from the ***_pts variable, you have the vertical position, and therefore a base to calculate the ladder step.
For furute application, please consider using a slider question with the ladder or stair scale.