[Suicide Junkie]

The 290 and 165 values are the break-even points of shields vs scattering and stealth armor, respectively.

If 15% (in the case of level 3 armor) of the total firepower the enemy must aim at you in order to breach your shields/armor is bigger than the break-even point, then the ECM effect of the armor is worth more than the extra shielding would be.

To figure out whether it is worth putting the armor on instead of more shield generator, do the following:
1) Take the shield points currently generated.
2) divide by the typical to-hit chance the enemy has against you. (30% accuracy means divide by 0.30)
The result there is the amount of weaponry/ammo the enemy usually has to fire at you to get enough hits for a kill.
3) Multiply the result in (2) by the ECM ability of your armor (0.05 = 5%, 0.10 = 10%, 0.15 = 15%)

If the answer you get is bigger than 165, then stealth armor is worthwhile. If it is bigger than 290, then scattering armor is also worthwhile.
If the number is less, then keeping the shields will make your ship Last longer.

Note: Odd situations will keep things dicey.
If the enemy has a super-huge gun (battlemoon, say) that kills in one shot, then you'll always want to go with the ECM, since hitpoints don't matter anymore.
Missiles and talismans will push the results the other way, since ECM is useless.