Initially both values are set to 0. To set the values, start raising the “high” value (in the lower field), and then set the “low” value. This is because the program won’t let you raise the “low” value above the “high”. This means that as long as the “high” value is set to 0, the “low” value can’t be raised at all! Not all operations use both values, and some use none of them.
Start by changing the “High” value...
...then adjust the “Low” value.