US dollars - move now or wait?

The closer I get to my move date the worse the US-UK exchange rate seems to become.  I have some money in an emergency fund and a fund with the start of a house down payment.  I won't need either of these immediately.  The emergency fund, hopefully never, but I could put expenses on my credit card and then access the money within 30 days.  The house fund, probably not for at least 12 months.

Does it make sense to set a lower limit for when to exchange and wait until the rates hit it?  For example, right now XE is giving me about $1.85/GBP.  What if I said I won't exchange until the rate drops to $1.67/GBP?  It seems like a good idea, but it also seems a bit like trying to time the market (buy low, sell high).  There is always the risk that the rate just rockets up from here and is a lot worse when we actually do need the cash.  Someone recommended that I have an upper selling point too - for example, if the rate hits $2.15/GBP we will exchange it all and cut our losses.

Does anyone have any thoughts on this?

My husband gets paid in USD, so we have to keep an eye on the rates. I've been here about 18 months, and I have never seen it hit $1.67, though I'm always hoping it will.  If you're seeing $1.75 or lower, that's a good time to buy £s.

I've also never seen $2.15, though I have seen $2.07. Talk about a sick feeling in your stomach!  ) Anyway, I wouldn't necessarily set a high price to sell at, as the dollar fluctuates quite a bit. It may take a couple of months, but it always seems to come back down to about the $1.80 range. So, you could stand to lose a lot taking the "well, I should cut my losses" attitude. . . .Even right now, the dollar seems to be in this type of cycle. It spiked to about $2.00 earlier this month, but now its at $1.81, and (knock on wood) seems to be coming down even further. When it comes down a little bit more, you better believe we will be buying as many £s as possible. 

You may want to consult a financial advisor, who can speak to your needs on this subject more.