The default right margin (accessed via Edit > Options > Editor) only permits a maximum of 5.58in - ie the width of the dialogue box. When you actually apply the default to a text file, it gives a width of 8.5in or so, but if you wanted to set a default right margin of between 5.58 and 8.5in, it is not possible to set using the editor default.
This has been mentioned a few times but so far I haven’t seen it acknowledged by a moderator (I could be wrong!).
One point, which may influence whether or not you wish to use this as you are, is that there is a big difference between margins, and the triangle controls on the ruler. These are indents, which are meant to be used as local offsets. Margins on the other hand are for handling whole document metrics, and are specified in your print settings, so if all you want to do is constrict the width of your document to 5.58, you should do so there, and not using the indents. Ruler indents are meant for localised offsets, like block quotes, lists, etc, rather than for impacting the entire document.
Second matter, due to how Scrivener counts 0 from the margin box and not the paper edge, if you’re using the 5.58" figure based on your experience with word processors (which count their rulers from the paper edge), you need to factor in the page margin width. Since a typical left margin is 1", that means the ruler is actually offset, and the right margin you really want is 4.58", not 5.58", which would actually be 6.58" (and outside of the typical margin block by a very small amount) in a word processor. So just keep that in mind when working with right-indents in particular.
The concern I’m raising is that when you attempt to set a default width for for document (I’m not talking printing margins here) using Edit > Options > Editor. Try this yourself in beta 023. If you try to increase the default width of the page by dragging the triangle indent to the right, it won’t let you.
I don’t want to constrict my document to 5.58 inches - I would like to set a wider default width - 5.58 inches is the maximum width permitted by the editor. This width is simply the maximum width of the Options dialog box.
Great, sorry if I didn’t make it clear, but I wasn’t attempting to say you didn’t have an issue spotted—just making sure you weren’t using it for print margins. You are absolutely right that someone might want a default right-indent of something greater than the current width. However it looks like you found another bug as well. What should happen if you drag the right-indent marker all the way to the right is that indent is just disabled. This is how the editor works out of the box. If your editor is 20" wide on a large screen, the text will not wrap at some point in the middle but go all the way across the screen. That is how things should work in 99% of the cases, stuff like block quotes excluded.
So there are two issues here. (a) You can’t have a global right-indent of say 6" if you want (most people will not need that though); and (b) you can’t disable right-indent, short of factory resetting your preferences as far as I can see. This works as expected in the main editor ruler, so hopefully a fix will be easy.