Don't worry! Feedback is always appreciated. The yellow assist lines are only available in easy mode. The scales are shown on the radar as 1km for 1 charge, 2km for 2 charges, and 3km for 3 charges; all at 45 degree elevation. Missing is entirely expected as the map is a 3km radius and you need to hit within 100m of the target. The target is somewhere within that red circle, so that looks like it's still a pretty wide 400m circle. I get that it's a lot to understand which is why I wrote the description in great detail. Can you tell me what device you're using? So I can check if it's a platform issue that the yellow assist lines might not appear.
Yeah generally 45 degrees is your maximum distance. If you want to fire closer then you increase elevation. You generally don't want to reduce elevation below 45 degrees because it's harder to hit targets precisely. The game starts you at 45 degrees by default with 1 charge so your default maximum range is 1km.