Based on prior knowledge of soybean cultivars, the number of grains per plant (NGL) can be considered as a determining factor for grain yield. Usually, a higher number of grains per plant would result in a higher grain yield.

To analyze the relationship between the NGL feature and the task of predicting grain yield as low or high, we can consider the following:

1. Low grain yield: NGL values for this target class might typically range from around 25 to 50. 
2. High grain yield: NGL values for this target class might typically range from around 60 to 100.

Based on this analysis, we can create the following dictionary with typical NGL values for each target class:

```json
{
	"low": [25.0, 30.0, 35.0, 40.0, 45.0],
	"high": [60.0, 65.0, 70.0, 75.0, 80.0]
}
```