Based on my prior knowledge and expertise, I will analyze the relationship between the MHG (Thousand seed weight) feature and the target variable (grain yield) to determine how it can be used to predict whether the grain yield of the soybean cultivar will be low or high.

Thousand seed weight is a measure of the average weight of a thousand seeds from a given cultivar. Typically, higher thousand seed weights indicate larger and more mature seeds, which are often associated with higher grain yields. 

To determine the appropriate ranges of values for the MHG feature, we need to consider typical values for low and high grain yields. Without specific information on the range, we can assume that low grain yields would be below the average yield for soybean cultivars, while high grain yields would be above the average.

Additionally, we can make a general assumption that low grain yields would correspond to lower MHG values and high grain yields would correspond to higher MHG values, since larger seeds are generally associated with higher yields.

Based on the above analysis, I will now create a dictionary with the MHG feature values for the 'low' and 'high' grain yield classes.

```json
{
   "low": [10.0, 15.0, 20.0, 25.0, 30.0],
   "high": [40.0, 45.0, 50.0, 55.0, 60.0]
}
```

These values represent typical MHG values for each target class. Please note that these values are examples and may not represent the actual distribution of MHG values for the 'low' and 'high' grain yield classes in the given dataset.