Her more detail about Yukari ability

First, Yukari must identify the boundary defining the limit of her own understanding of the ability to manipulate boundaries. Carefully expand this, monitoring for the effects it is having on Herself until she reaches an understanding of what would happen if she completely removed that boundary. Assuming she finds that it results in an absolute understanding of boundary manipulation, with no side effects, Yukari can just remove it completely. Otherwise, just keep steadily increasing her understanding until She is satisfied with it. Yukari can always expand on it later if needs be.

Then, Yukari must repeat the process with the boundary defining the limit of competence with her ability to manipulate boundaries. \

Again, if Yukari's get new understanding gives the knowledge that completely removing the boundary would give Yukari absolute competence with boundary manipulation, with no side effects, then that would be the easiest way to do it.

Finally, Yukari would repeat the process again with the boundary defining the limit of how intuitive Yukari's ability is for use.

So at this point, Yukari...

Have an absolute understanding of boundary manipulation. The important part here is that she would have an understanding of the best way to achieve any desired result, without causing any side-effects. Have absolute competence with boundary manipulation. This means that, once Yukari has identified the best way to achieve a result, she can carry it out with a 100% success rate, and can never mess up the use of her power. Yukari Has made her power absolutely intuitive to use. Whilst Yukari still goes through the whole process every time she manipulates a boundary, Yukari no longer needs to think about each individual step along the way. She just decides to want to do something, and use boundary manipulation to achieve it. (Think of it like walking - once you learn how to walk, you no longer need to think about the (actually kind of complex) series of muscle contractions and relaxations necessary to perform the action, or the modifications you need to make to that series in order to avoid an obstacle, avoid stepping on something, or to adjust to compensate for walking along a slope. You just do.)

Since it seems like relatively simple boundaries to manipulate - expand or contract the boundary to increase or decrease the quantity or remove the boundary entirely to make the quantity limitless. This is as opposed to attempting to manipulate a boundary such as that between what is and is not real, where 'is not real' would contain a literally infinite number of things. All it would take is one slip up to cause the end of the world by bringing an infinite number of things into existence, or accidentally causing everything that 'is real' to become 'is not real '. Yukari must have absolute understanding and competence with her abilities before she even starts trying to manipulate boundaries that contain/withhold an infinite variety of stuff. Because She gains the experience of Yukari, Akira can use this ability with ease.