It's very important in type design to make sure all your letters are the same height. Otherwise, subtle irregularities might accumulate, and you might not notice if a letter is getting taller or shorter than it should be. This is why you should set up fontforge guidelines early in your design process.
The most important guideline to set is the meanline. This is the height that most of the lowercase letters should reach to. The meanline should align with the top edge of the lowercase ‘v’ or any other flat-topped lowercase letter. You set the meanline in fontforge by dragging down from the ruler at the top of the glyph window. Make sure you round the guideline to an integer!
You should also set the over and undershoot guidelines—use a round letter like ‘o’ or ‘e’ to determine where it should go.
The round parts of every curved letter—that includes the ‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘g’, ‘h’, ‘m’, ‘n’, ‘o’, ‘p’, ‘q’, ‘r’, ‘s’, ‘t’, and ‘u’—should touch these lines. So should any pointy letters like ‘v’ and ‘w’, which should have an undershoot, and any letter with a head serif including ‘i’ and ‘j’, which should have an overshoot. In theory, pointier shapes should under/overshoot more than curved shapes, but the bevel on the vertex of the ‘v’ makes this necessary. However, I do have my sharp head serifs extend three font units
beyond the curve overshoot.
You should also put guidelines into place for the ascenders and the descenders, and any other useful heights.
