1. First, You need to know that the ring is only 3 mm width then the font will height only 2.5 mm.
2. Use all upercase or all lowercase. If you use both, the lowercase characters will be a half (1.5 mm). It’s not easy to see it.
3. Considered between black or white font color. Surely black color will more visibility.
4. The font styles. The thin font make you can’t see it even you use the black color.
5. Wording legth, the long word make the font smaller. I suggest not over 10 characters for a white color.