I have added a one-shot button to a Perspective view and the property to change the icon position is missing. Additionally, the documentation shows the icon on the left, but for some reason, my version of the Ignition (8.1.26) insists on placing the icon on the right.
There's no avenue for declaring where the icon should be placed. We do have an open feature request to add such a property. I've "linked" that ticket in the title of the thread.
While it might not get you exactly where you want to be, you can at least get a portion of the way there with CSS:
You can also add in left/right padding values to tune it further. (There are definitely other options to tinker with that I'm missing)
Edit: Looks like
justify-content gets overridden outside the Designer. Can use a custom style and the stylesheet resource to set the