If an HSA is truly what you have, then remember that you get to keep, and carry forward, anything that you don't spend this year. And you don't pay taxes on it (up to a limit determined by your status as an individual or family). So its kind of a no-lose situation.
If instead you are referring to a Flexible Spending Account (FSA) then my approach is usually to go conservative. But even then, you should be able to get a sense of what an IUI will cost you out of pocket. I would add NonIUI/IVF costs + cost of 1 IUI OOP+250. That might likely put you at/over the limit.