If you go to the margins tab of the page setup dialog and select bookfold or mirror margins from the multiple pages drop down and then try increasing the gutter and then the inside margin you will see what affect each one has.
How to set createspace gutter.
You can go back and make changes if you need to.
For createspace you need to use the following measurements.
Lastly set up the spine.
If you re using mirror pages the value for the inside margins will be set in the gutter field.
Page extent minimum inside margin inches minimum inside margin mm 24pp 150pp.
Look at the back of paperback books for ideas and be careful to leave space for the barcode and trimming as per createspace instructions.
So if you want the inside margin to be 0 5 for all pages add this value in the gutter margin box and leave the inside margin field blank.
The gutter is in addition to the inside margin.
The minimum size of the gutter varies according to how many pages there are in your book.
The gutter margins need to be sufficient so that the spine or the readers eyes are not strained while reading.
The spine of the book is calculated based on the number of pages and on the type of paper you plan to use.
If you want a black white book published on white paper with up to 150 pages the spine will be the number of pages multiplied by 0 002252.
These both mean the same thing so you only need to set one.