I recently began making a new font. Unfortunately, I forgot to pick a document preset before starting. On the Glyphs documentation, it says that cap-heights are usually “between 600 and 700” units tall. Moreover, the ascender line usually sits at about 800 units.
My current set of capital letters is 800 units tall. I plan to put the ascender at 900. I assume that this deviation from the standard wouldn’t matter as long as the vertical metrics are consistent throughout every master in my font. Is my assumption correct? Will this choice cause any problems for future users of my font?
The size of the cap-height and ascenders relative to the UPM determine the visual size of the compared to other fonts. If your fonts looks too big, you can increase the UPM a bit (e.g. to 1100) to make it appear smaller.
I see. If I understand you correctly, there should be a 1:1 ratio between the height of the em and the UPM. For example, if the descender is at -200 and the ascender is at 800, 1000 UPM would allow appropriate scaling. Is this the idea that you’re getting at?
Appropriate is what looks good. You have to test this and compare with different fonts. Adjust the UPM until it looks good. Then you can scale everything back to a UPM of 1000 if needed.