I have this definition in the config file in 1.12:

    S:customTips <
        Powderkegs pack boom into one block with 100% drops...Go find that sulfur
     >

This results in a Format Error displayed:

Is there any way to avoid this other than doubling the %? If I do so, other mods displaying tips now need different text than the tips for this mod.

The tips are formatted using Mojang's text formatter. They reserve % as a special character so a double % is required to escape it.

Under normal circumstances I would look into adding a feature to auto-escape % however this issue is not relevant to modern versions and 1.12 is not supported.