Just have it in the random chance to land.
A one in six chance, that NS is triggered on Key release, if you're not already on a GCD.
With spamming, they'll fire off fairly randomly and regularly.

With four DK's spamming away...
After the initial Howling Blasts, you either blow runic power on Frost Strikes, or do Blood Strike/Necrotic Strike.
You'll average one Necrotic Strike per DK, every 5 seconds.
Which is plenty of heal reduction.
No need to micro manage the ability.
Just mindlessly spam your key, and trust that you'll have NS up on the target most of the time.