The percentage of Deku's power usage changes based on a lot of factors. Like the nature of the enemy, his training progress, and the urgency of the situation. So, it's hard to give a fixed percentage.
In the 'harry shows his power fanfic', perhaps it's a story where Harry has to show his power in a wizarding competition. He might use his unique abilities like Parseltongue to outwit his opponents and prove his worth. He could also draw on his connection with the Patronus charm to protect himself and others during the tasks.
There are fanfictions where Naruto's power is shown through his ability to connect with the spirits of past Hokages. With their combined wisdom and his own strength, he's able to perform a super - powerful jutsu that not only protects Konoha but also restores the balance of the ninja world. He uses this new - found power to face an ancient evil that has been awakened.
In this fanfic, Harry is the main character. However, we could also expect to see Dumbledore making an appearance. Dumbledore has always been a guiding figure for Harry, and in a story about Harry showing his power, Dumbledore might offer some wisdom or crucial information. Neville could also be a key character. He has his own growth in the original series, and in this fanfic, he could be inspired by Harry's power and contribute to the overall plot in some way.
Well, Naruto could show Boruto his Sage Mode power. Sage Mode gives Naruto enhanced strength, speed, and the ability to sense chakra more acutely. It's a very powerful form that Naruto has mastered over time. By revealing it to Boruto, he might be teaching Boruto about the different levels of power and the hard work required to achieve such power.
In fanfictions, Naruto's power growth can be much more rapid. For instance, he might quickly master new jutsu that in the original series would have taken a long time to learn. This is because fanfiction writers often want to show Naruto at his peak or explore different power - up scenarios faster.
Bakugou could use his words. He might tell Deku that he believes in him. For example, he could say 'Deku, you may be a nerd but you're a strong nerd.' It's a backhanded compliment but it shows he has faith in Deku.