Thanks Artur! Yes, I capped it: letters never go below 15 mm, so the spiral stops before the centre instead of shrinking into an unreadable dot. I tested a few sizes on actual fabric, because ink spreads more on cotton than on paper. The centre ends up holding a small motif instead of text.

Testing on real cotton is the part most people skip, ink gain changes thin strokes a lot. Putting a small motif in the centre sounds like a neat way to turn a constraint into a focal point.