I'm trying to edit a version of the Tanakh, and I was wondering if formatting the text with justification (so that it spreads out evenly across the page) is permitted.
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I have never heard of a restriction. Most (if not all) copies of Tanach I've seen are justified. In fact, copies of the Pentateuch that are written in the traditional style (handwritten on parchment, etc.) should be justified (except that some sections should end mid-line) — but that rule doesn't apply to print, paper copies.
Note, though, that, like Wikipedia, this site makes no guarantee of validity, and does not offer professional (particularly rabbinic) advice. Treat information from this site like it came from a crowd of your friends and consult your local rabbi for practical advice.
 By implication from Maimonides, Sefer Tora 8:1–3, Shulchan Aruch Yoreh Deah 273 and Keset Hasofer.
Some places in the Torah need special format such as the 10 Commandments and the Song by the Sea. There is also a concept that some paragraphs are supposed to end on full lines and others are supposed to end in the middle of a line and the next paragraph begins on the same line (and there is supposed to be a gap).
I don't know if these rules apply to the Nach parts.
There is, however, some special format in Megillas Esther see http://ohr.edu/5375
These special formats may disagree with justification.
As long as you put a disclaimer in that this book is not for ceremonial purposes, I think you will be all right