I had good luck with .5.


Betsy Ross wrote:
I have a miserable time writing on fabric with my Micron pens. In
another thread, others suggested sandpaper to hold the fabric. I have
yet to try it.

If the quilt is already completed, it will be very difficult to write on
especially with the fine tip Microns. Perhaps a broaded tip would work.
I would try writing on a sample of the same construction.