I posted a long time ago about the special characters that control the three types of stacked fractions in multiline text: horizontal, diagonal, or tolerance (no dividing line). That post made brief mention of the AutoStack settings dialog, but didn't explain it, so I thought I'd come back to it now.
The first time you ever type a fraction-like characters in an mtext object, the AutoStack dialog appears to ask you how you want those characters to be treated.
The first choice to make is whether to turn AutoStacking on at all. If you don't want it, all fractions will stay exactly as you type them out, like this:
Next, you can choose to condense complex fractions by removing the space between the whole number part and the fractional part. Depending on the font you use, one way probably looks better than another:
Most people type out fractions using a forward slash (/) as the divider, but in LT's world, this could be displayed as either a horiztonal fraction or a diagonal, so you get to choose. You can see in the above image the difference between the two types.
Lastly, you can choose whether to have LT remember your choices or to prompt you with the dialog every time you type something that might or might not be a fraction. If you use a lot of fractions, you'll definitely want to check the box.
If you change your mind later, or if you want to unstack something that shouldn't be stacked (dates like 3/4/2009 get interpreted as fractions too), select the fraction, right-click, and choose Stack Properties.
Here, you can edit the upper and lower values of the fraction, adjust its style (diagonal, horizontal, tolerance, or decimal, which is a variation on tolerance), its position in the line of text (at the top, center, or bottom), and its size (as a percentage of the size of unstacked text). The defaults for Position and Text size are Center and 70%, and I find that that works well in the vast majority of situations.
The Stack Properties dialog is also where you can find the AutoStack settings after that initial pop-up.