The Mythic + build from Wowhead and icyveins are essentially the same, which doesn’t include the Shiv talent (2 charges). So the question is, should you use shiv before kingsbane or after (only 1 shiv charge)? The assassination rotation goes, it should be:
If you look under the cooldown sequence on the wowhead guide, it shows the exact button presses and shiv timings. The shiv right before kingsbane is only if you are playing double Shiv build. Otherwise the optimal timing is where the second shiv is to amp the end of kingsbane.
If you are playing the aoe shiv/bleed buff talent you can use one shiv between kingsbanes to buff a big bleed aoe pull, but you should be aiming to have shiv off cd for the last 8 seconds of kingsbane as often as you can.
Like @Weran said, with one charge, Shiv should be used for the last 8 seconds of Kingsbane, since that’s when nature damage is at its highest.