Looks like a couple of my posts have gone missing. Weird. Let me save you a lot of time and hassle regarding the use of a ruling pen: Get one. It is very easy to use for small straight lines using watercolor. I found mine at our somewhat local university bookstore. While I was there I picked up a rolling ruler which was competitively priced and a package of french curves (included three). Also purchased a three sided eraser that is easy to hold.
To use the ruling pen, I mixed together a generous amount of burnt sienna and water together in a small gerber juice bottle and shook well. Then I just dip the pen so that it retains an average amount then start drawing. It helps to hold the pen somewhat on its side, not straight up and down. I also recommend taping pennies (3) to the bottom of your french curves or triangles if they don't sit up from the paper as it could prove a problem with paint getting underneath. I still have my original triangles with pennies taped to the bottom from drafting and architectural design classes I took many years ago. I haven't even had to change the tape.
I am doing a value study of a couple of tractors, but will be helping judge some yearbooks today and won't get back to my value study till this evening.
"Spring has returned. The earth is like a child that knows poems." - Rainer Maria Rilke