Use a soft pencil. 2B is my preferred hardness with a reasonable balance between softness & durability. Definitely not anything in the H series, not even a HB. A soft pencil will rub off without leaving a mark. If needed wet your thumb/finger with a bit of saliva then rub.
Slight change of topic, but you can save some money & use a Jeppesen CR type whiz wheel. Every bit as accurate as any slide type whiz wheel but more compact and, for some problems at ATPL level, quicker to solve with fewer steps than needed with a CRP5.
Lots of UK instructors don't like the CR based on lack of familiarity but they're not hard to use.