By the way 'chinagraph pencil' is termed a 'grease pencil' in the US, I gather.
Those wretched pens with removable tops leave you either struggling to fly the thing with your knees with pen top in one hand and pen in the other! Or with the top stuck in your teeth. Use a 'topless' grease pen and you'll find it a lot easier.
Pink_aviator's suggestion to know the equivalent length of your thumb on the map (about 10 nm) is a good one - alternatively paint some mile marks on your pencil.
There's nothing like a cocked-up 'pre-test' to get you working on your techniques!