Unholy Death Knight Frequently Asked Questions — Dragonflight 10.2

Soul Reaper should be used off cooldown if the target's health is 35% or below. You can also use Soul Reaper before the target hits 35%, as long as the target will be 35% or below when the Soul Reaper debuff expires. It is important to save a Rune for Soul Reaper so that you can use it as soon as it comes off cooldown!

At 68.4% Mastery;

No. The procs stack on top of each other. Any remaining duration is simply added to the new one if you get another proc.

Yes, it is the go-to build everywhere.

There are not any real breakpoints to aim for, generally get as much Haste as you can for single target (raw amount less than Mastery), but also trust your simulations!