If you re using mirror pages the value for the inside margins will be set in the gutter field.
How to set createspace gutter.
For createspace you need to use the following measurements.
If everything looks okay submit your files for review.
The minimum size of the gutter varies according to how many pages there are in your book.
Lastly set up the spine.
The gutter margins need to be sufficient so that the spine or the readers eyes are not strained while reading.
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.
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.
You can go back and make changes if you need to.
Look at the back of paperback books for ideas and be careful to leave space for the barcode and trimming as per createspace instructions.
Review your project setup.
However traditionally gutter spines have also been the smallest of the margins since the recto and verso pages laid together each with a large gutter spine will create an unpleasingly blank look to the center of the book.
The spine of the book is calculated based on the number of pages and on the type of paper you plan to use.
The gutter is in addition to the inside margin.