Put 8 0z. in a metal cocktail shaker and put it in a bowl of ice water. Swirl it around. In a minute or so, it will be in the 60s and you can take an accurate reading.
That's a good idea. I'm thinking I could add my -60min hops, boil for 30min, check gravity, if it's low, plan on boiling longer. I don't think you'll get much more bittering at -90min vs -60min. If it's too high, I could add the top-off then. Sound good?
Woops, Major beat me to it.