Based on prior knowledge, the number of grains per plant (NGL) can be a good indicator of grain yield for soybean cultivars. Generally, a higher number of grains per plant would result in a higher grain yield. 

Here is a dictionary with typical values of NGL for each target class:

```
{
	"low": [5.2, 6.0, 4.8, 5.5, 5.3],
  "high": [7.9, 8.2, 7.6, 8.0, 7.5]
}
```

These values are just examples and can vary based on the specific cultivar and environmental conditions.