In Expert Mode, you can create as many guides as you wish. So you can keep your ascenders, descenders, x-heights and letter widths consistent.
Just rollover one of the green-dotted guides and a clickable plus sign should appear. Click on that to create a new guide. If there is more than one guide of a given orientation, a minus button to remove a guide should also appear on rollover.
If nothing's happening, remember to check that “Lock Guides” is not ticked in the menu.
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