Best way to handle mid-iteration deadlines?

I often have release deadlines that fall in the middle of iterations. Our release dates are dictated by the client and fall at fairly irregular intervals, so it's difficult make them all land near the end of an iteration period.

Tracker appears estimates your ability to meet a release based on when the iteration it's in ends -- i.e. it does not "prorate" your velocity based on the release deadline. Nor does it apparently allow for partial iterations or the like.

Given that, what's the best way to handle these sort of things? I'm currently just prorating velocity in my head -- e.g. if I have a one week iteration (with a 5-day work week), a velocity of 15, and a deadline that falls on day 3 of my iteration, then I know I'm going to be losing sleep if I have more than 9 points of work to do. Is there a better recommended way to do this?