The biggest problem Calls have when hatching is that with their head tucked under a wing their bills are too short for effective pipping and zipping. The result is that they die in the shell at some point without hatching. I have used a tack to poke a small air hole at lock down to increase the chance they will survive longer between the internal and external pip. This buys time if they need assistance zipping before suffocating to death.
If the duckling is unable to pip the shell or zip it open to hatch, you will have to do a slow assist. Let the duckling struggle enough to build up its muscle strength without becoming overly exhausted. I think hens have a more intuitive sense of how to assist during hatching but you can kind of follow your own intuation based on your experiences with a normal hatching timeline. The trick is not to go too fast or too slow.
There are lots of articles written about assisted hatching so I don't want to write it all out when my posts are already longer on average than most. The challenge is doing a slow assist while a hatching bird should be in lockdown for the entire hatch. Every time you need to peck away bits of shell, it can change the hatching climate with the correct temperature and humidity you need to maintain. Hens only have one temperature setting so they just plop their downy bottoms down on the eggs and their body heat and moisture does the rest. In an electronically controlled environment it becomes harder to keep the hatching environment stable.
The other curve you throw into assisted hatching is upending the baby inside the egg. I am taking meds that make me dizzy just by rolling over but after a moment of disorientation I can figure out which way is up. A baby that has no concept of gravity or that up is up and down is down may get too "dizzy" to know which way to hatch. If the eggs are pointed end down in the incubator, keep them pointed end down when handling them. If they are on their sides, make sure the air cell slant is up and stays up. Hold the egg in place while you assist in helping break the duckling free without turning the egg around to better see what you are doing.
Another important thing to remember is that you are only compensating for the duckling's short bill when hatching. Do not pop the cap off the shell, rather let the duckling push out with its feet as if it had zipped on its own. If you help too much you will have a weak duckling that will flop around on a flat surface without first pushing against the egg shape it is accustomed to its entire life. There is a succession of rapid development that takes place during hatching and rushing the process will not allow the baby to develop strength properly. Infants prop up their heads, roll over, crawl, sit up, pull themselves up, balance on their feet, then they walk. Ducklings walk so much faster but they still need to develop the correct strength first. The duckling needs to do as much as possible without help from you or it will be too weak to survive.