I believe that if you wear anything deliberately trying to have people lust after you, you are creating a stumbling block for them and causing them to sin. Anything that you wear on purpose to make the opposite sex lust after you or the same sex be jealous of you is against a biblical lifestyle and i believe that is how modesty should be determined
I disagree with your application of the "stumbling block" teaching, because in Romans 14, Paul said that it was "causing to stumble" if someone sees you doing something and they join you in doing it, despite the fact that they think it's wrong. In other words, if a strong Christian woman wears a bikini, and her not-so-strong-Christian friend sees her and decides to wear a bikini, too... because her strong friend did (even though she kinda thinks it's wrong), then--and ONLY then--the strong Christian woman has caused her weaker friend to "stumble."
It is NEVER "causing to stumble" if someone has a sinful response to another person who is just living their lives. Remember, there were a LOT of Pharisees that had a sinful response to Jesus' actions... all the time. Yet it was not a sin of Jesus that they had that wrong and sinful response to Him.
But let me ask you about your measure of "intent" being the measure of "modesty." If that is so, then a woman could be modest in a bikini at the beach, since that sort of attire is accepted there. And she may go to the beach solely for the purpose of recreation and catching some rays... completely
without any intention at all to create lustful responses in others.
And going further, on many beaches in the world, it is perfectly acceptable for women to go topless. So--again, by your measure--if a Christian goes topless in that setting (with no intent of inflaming lust), then she too would still be "modest."
For the record, I know that's probably not what you meant, but I would agree... that when someone intends to cause lust in others, there's a problem. But if not--and the context is such where it is appropriate--then it is NOT "immodest"... even if the women is topless.