csRuler allow meassure the distance betwen 2 points/nulls and get a visual feedback for these distance (also are a parameter in case you want use it in some expression).

Once installed the addon you can add csRuler in your scene form Get -> Primitive -> csRuler, with the addon comes a little video showing how to use it :)

I hope this help, if you have any comments or find a bug please send me a message and i will try to fix it.

Cheers!