> how may I judge it's level of reasonableness?

By looking what a straight line in UTM looks like on a curved grid (such as a rotated pole etc.).

e.g. 

```
plt.axes(projection=ccrs.RotatedPole())
plt.plot([x0, x1], [y0, y1], transform=ccrs.UTM(zone=30, south=True))
plt.show()
```

Picking a line which would have a reasonable amount of curvature in the target projection will give you an idea of how good the 1e4 threshold is in this case (if the line is rather jagged, then it needs to be smaller).