I think that it's better to have a consistent minimum than a consistent maximum. This is because our time and energy levels aren't consistent.

Pick the smaller number for your "consistent" number. Something attainable, even on the bad days. Go for 25, if that feels good to you. However, if you feel like exceeding that, this is perfectly fine as well.

Consistent minimums are perfectly fine to encourage you to continue to read. I don't recommend consistent maximums, however. If you feel like reading more, then let it happen.