Like many "simple" tools, there's more than meets the eye.

1. you need a "fence" to register on the edge of the stock
it should
- be flat/straightt and square to the "arm" with the line
cutter/ scratcher/ scriber
- be precisely moveable
- lock in place without going out of square or moving
from where you started to lock it.
2. you need an "arm" to hold the cutter/ scratcher/ scriber
which should
- stay square to the fence face
- hold the cutter/ scratcher/ scriber parallel to the
fence face
- hold the cutter/ scratcher/ scriber square to the
stock
3. you need a cutter/ scratcher/ scriber which should
- sever grain rather than scratch/scrape it
a simple pin scratches and can follow the grain,
wandering off line if you file a flat bevel on a pin,
parallel to the fence to create a cutting edge rather
than a point AND round the "point" to create a curved
cutting edge. it will severe not scratch and is less apt
to follow the grain
Better yet, use a small triangular knife ala the
japanese marking gauges
Best of all, a single bevel cutting wheel - as on the
Tite-Mark and the LV/Veritas marking gauge

Things get a lot trickier if you want to make a mortise
and tenon marking gauge. THAT'S where the Tite-Mark
with the optional dual single bevel wheels really shines.
Set them on the shaft to the desired width, lock them down
then set the fence to position the mortise relative to your
reference face.

More stuff to think about - sorry.

charlie b