Based on my prior knowledge and analysis, the feature NGP (Number of legumes per plant) can be a good indicator of the grain yield of soybean cultivars. 

Typically, a high number of legumes per plant indicates a healthier and more productive plant, which is likely to result in a high grain yield. On the other hand, a low number of legumes per plant may indicate a weaker or less productive plant, which is more likely to result in a low grain yield.

To generate the dictionary, here are five typical NGP values for each target class:

```json
{
	"low": [2.5, 3.0, 3.5, 4.0, 4.5],
	"high": [6.0, 6.5, 7.0, 7.5, 8.0]
}
```

Note that the values provided above are just examples and should be adjusted based on the specific data and characteristics of the soybean cultivar being analyzed.