Sometimes the old simple methods work better than electronics. The more familiar you become with your airspace, the easier it will be to estimate track distance......however for absolute simplicity I still use my pen.....just hold the pen flat against the display over a known distance [make a mark on the pen if you use the same displayed range all the time]. Next time you get a request it is the quickest way to provide an answer.....and in my experience the pilot only wants a rough guide.
Lost pen? No problem...use your finger flat against the display.....advantage here is that your finger can bend and accommodate circular or not-straight section distances.
Other option.....your employer could spend several millions for an electronic tool that does the same thing.