Unplug the cable at both ends. Then run a piece of string via the route you want the string to take. Add a couple of centimeters to the length of the string to ensure there's enough slack. There's your measurement.
This tends to be the best practice way of measuring for custom cables. Or take a look on r/SFFPC for measurements for your motherboard and either this case, or the Ncase M1 (worth adding a couple of cm to M1 measurements as the PSU in the NR200 is slightly further from the motherboard).