I learned about controlling width of TOC columns from https://support.bible/12103/formatting-toc. This was helpful for professional-looking Front Matter. I discovered I could size the columns more precisely by setting the Left Indent Factor to a positive number and the First Line Indent Factor to a negative number. I've replied to that topic with details of what I did.
But tweaking the Indent Factors seems only to work for the complex \cat:toc styles. I can't seem to adjust column widths in two static tables in the Front Matter, which list the language's OT and NT book names.
My tables in the screenshot are centred on the page but off-balance with each other. It would look much better if both tables had the same dimensions, but tweaking the Table Column styles (\tc1...\tc4) Left... and First Line Indent factors has no effect on the column spacing, and the two tables align very differently (whereas the cat:toc styles apply to all TOCs in the publication, I believe.
It would also look much nicer if the "gutter" between the two tables were centred, that is, if columns 1 and 3 were the same width. The long rendering of Revelation causes the NT table to look lopsided.
Hmm, could I specify the tables as 2-column sections? With the second title as a single-column section? Don't know if this is possible in a Front Matter file.
Sample code for the tables:
\tr \tc1 Uakeken \tc2 Uak \tc3 O Uelhireng Tar Niate Uaia \tc4 OUNU
\tr \tc1 Kale Tapokis \tc2 KT \tc3 No Kereker Solomon \tc4 NKS