Did a search and didn't see anything about this, so thought I would chime in. Noticed at least one post where mention was made of "dusting off wrong chalk marks" during kilt construction. My sister clued me in to products such as this: http://tinyurl.com/55457u which as it states is a tailor's chalk made specifically for use on wool. Its composition is such that it disappears over time or -- and this is the cool part -- when steam-ironed! The link given shows it sold by the big box but I'm sure you can find "personal quantities" at local shops or elsewhere online. There are several brands of similar purpose, I don't know specifically which one my sister uses, just Googled "disappear tailor chalk" to find this example.

Hope it helps someone down the road.

P.S. There's also a variety that disappears to the naked eye but still shows under black light. You supply the joke