Kindle Scribe - change right & left margins?

I just got a Kindle Scribe and have found the left and right margins need to be smaller.

I have spent about two hours now reading forums on here about changing these margins in the conversion process on Calibre and I cannot find any that have worked for me.

What is the best way of making sure all Calibre conversions have a set left and right margin (for example a negative value)?

Thank you!

EDIT:

All good - worked it out