The “Image with text overlay” section in Narrative theme use padding to determine the central element position (only button in your case) using the following code.
@media only screen and (min-width: 750px) {
.hero--large .hero__inner {
padding: 220px 0;
}
}
The “Section Spacing” option determined the padding used, for Large is 220px top and bottom.
You can adapt your code by defining your custom padding and add it at the end of file theme.scss
@media only screen and (min-width: 750px) {
.hero--large .hero__inner {
padding: 400px 0 40px;
}
}
For example in this case I set 400px padding for top and 40px for bottom.
You have to tune those values based on your preferences.