I usually set up as three columns, and don't worry all that much about adding guides (they won't be in the print). You can add a slug and put folod marks in it if you like, but in my experience it won't really matter since they'll be gone, too by the time the bindery guy sees the brochure (if it was actually printed on stock large enough to hold them), so he's gonig to set up the folding machine to give you a good fold, whcih usually means the two outside panels will be a bit wider than the one that folds in. The difference is usually small enough that you don't need to adjust the panel widths unless you want to. I generally set the gutter between columns to twice the outside margin. .1875 seems pretty light to me, though.
I think that answers my question. I needed to adjust the gutter size instead of trying to add a guide.
I set my margins but without the guide my type was not centered in the panel. By setting the gutter twice the size, it is perfect.
You mentioned that .1875 was light to you... If I may ask, what do you usually set your margin at? I appreciate all the advice you can give me
I would use .25 minimum, and probably .375 or maybe more, if I could, especially if the printing is going to be done on something other than a press (most digital equipment needs .25 to .3 around the edge, and you want a little margin for shift on the sheet because page-to-page registration will float on digital printers). the wider the margins, the less noticeable a fold line that is a little off-center in the gutter becomes, too.
If space is really tight, you can use a narrower gutter than a full doubling of the outside. I LIKE seeing an even frame around the panels on either side of the fold, but there are certainly people who would say the space between columns should be the same as the outer margin so it looks even when laid flat. Do a couple of mock-ups with different gutters and fold them to see what appeals to you.