Based on prior knowledge, the feature "Number of grains per plant" (NGL) can be a good indicator of grain yield in soybean cultivars. Generally, a higher number of grains per plant would correspond to a higher grain yield, while a lower number of grains per plant would correspond to a lower grain yield. 

To generate the dictionary with specific details, we need to provide 5 typical values of NGL for each target class ('low' and 'high'). Since the specific ranges of NGL values were not provided, I will provide example values based on common ranges for soybean cultivars.

```json
{
	"low": [80.2, 85.6, 81.3, 78.9, 83.1],
	"high": [105.4, 102.7, 108.9, 104.3, 106.2]
}
```

Please note that these values are just examples and may not represent the actual ranges of NGL for the 'low' and 'high' target classes accurately. The actual values would depend on the specific characteristics of the soybean cultivar and the given dataset.