You are correct in believing that you do not need 490 Defense to tank most trash. The formula is as follows:
Base mob crit = 5%, modified by 0.04% for every point of difference between their Attack and your Defense.
Mob Attack = 5 * level. Therefore, a level 70 mob has 350 Attack, a level 71 has 355, a level 72 has 360, and a level 73 has 365.
Doing the math, to be crit immune you need Defense = [Mob Level] * 5 + (5 * 25). Plugging in the numbers gives us:
Level 70 = 475
Level 71 = 480
Level 72 = 485
Level 73 = 490
Most raid trash caps out at level 72, so you'll need 485 Defense to be safe. The occasional level 73 mob will still have a chance to crit you, though.