Based on prior knowledge, the number of grains per plant (NGL) is generally positively correlated with grain yield. Higher values of NGL tend to result in higher grain yield, while lower values of NGL tend to lead to lower grain yield.

To create the dictionary, we need to provide a list of typical NGL values for both the 'low' and 'high' target classes.

Assuming that the grain yield is considered 'low' when it falls below a certain threshold and 'high' when it exceeds that threshold, we can envisage the following typical NGL values for each target class:

```json
{
	"low": [300, 350, 400, 450, 500],
	"high": [600, 650, 700, 750, 800]
}
```

These values are just examples and may vary depending on the specific context of the analysis.