That using a Flask of Chromatic Wonder will allow the tank to keep his/her normal tanking gear on for at least one of the slots.
Note that it does-not-matter WHERE the stat comes from (gear, food, flask, etc.) or which gear slot has it. All the combat table cares about is that [insert stat here] is there somewhere. That said, a tank would be over-gearing beyond 260 gear resist if the tank will also benefit from a +70 resist buff and +35 resist from Flask of Chromatic Wonder.
Also, as an example only, the spell damage reduction tiers are:
75% = 365
60% = 292
45% = 219
30% = 146
15% = 73
The tiers are examples and not absolute, so anything between 292 and 364 will still provide additional damage reduction. Also, according to wowwikki, the chance to resist is "25%, 50%, 75%, or resisted completely or not at all".
Resistance - WoWWiki - Your guide to the World of Warcraft
Nice post, BTW!