[Celeal]

IMHO, it is situational: It depends on the tank's skill, mob and party support. Things get ugly when the blink tank is interrupted while casting shadows. It may get out of control if the main-blink tank is interrupted multiple times in a row.

As a backup tank, besides counting your main tank's shadows, you may also aware of casting interrupted message from your blink tank (for all Ninjtisu casting, including those enfeeables from NIN). It is because party's help is critical when the blink tank is not able to renew his shadows.

Counting your main-blink tank's shadows (from a backup tank point of view) helps, but you have to aware that sometimes your NIN maybe able to evade the mob's attack without shadows (with enough evasion, debuff, and luck), or the mob randomly double attacks, attack->WS->attack back-to-back, multi-hit attacks, Aoe... load of stuff that can screw up counting if you are not alert.

Watching the blink-tank's HP helps too, but you should as aware that some mob's attack ignores (go-thru) shadows without dropping the shadows, such as Goblin's Bomb Toss or Crawler's Fan-shape breath attack, etc.

From my experience, watch out your blink-tank's "casting interrupted" message and support accordingly (back-up provoke, stun, etc) is easier for new players.