Hello flythisway
First of all, good luck for the Nav Ex
It has been a long time since I did my PPL Nav Ex, and a few years since the ATPL Nav Ex
My personal advice is bin the calculator and practise the Whizz Wheel.
Mental arithmetic and the mastery of the WW is key to passing, electronic calculators can be a hinderance.
The Whizz Wheel can produce all the answers you need and above all it will never produce a 'button error' or crash through low batteries.
In the ATPL exam there is a bank of questions based upon the whizz wheel, and you need to be super adept at operating it to pass, believe me you get cramp in your thumbs and fingers after.
As for writing on a chart, I normally use a fine watersoluble OHP pen, similar to the Staedtler Lumocolor, or a soft pencil as Mike744 mentioned.
Once again, good luck!
FB