Once I've converted to KF8, I load that into the eBook editor and if I am happy with the margins, I add in the snippet that puts in the negative margins. I do it in html and it work very well.
Code:
html {
margin-right: -45px;
margin-left: -45px;
break-inside: avoid !important;
}
The break-inside line fixes an issue with centering with KFX using one of the Calibre plugins to generate KFX.