How can I fan out my magazine tree more evenly?
If you are willing to do the whole tree one more set of folds, you can fold the pages into narrower triangles which will each be thicker and as such will fill out the tree more.
It might be best not to do all pages but skip three and do it a second time for the middle of the three left if more work is needed.

This will alter the look of your tree, making it narrower, or making it less even all over.

An alternative that does not change the part of the tree you see is to add a trianglar block in the back, to fill the hole.