Volume measurements are more consistent than weight measurements for spruce tips because the moisture content of the tips can vary widely. Use a quart or so (you can use less if you want) in the boil for 60 minutes - sitka spruce tips will give you a delicious citrusy flavor, douglas fir tips are similar. Not sure about other trees.
Tom, are speaking form experience with a 60 minute addition? If so, which brewery's product would you compare it to? Have you or anyone you know, experimented with other addition techniques? I.E. First Wort Sprucing, late, whirlpool & Dry Sprucing. I plan on doing my first Spruce ale next summer when the tips are fresh in NE Wisconsin. Until sound advice convinces me otherwise; I plan on doing spruce the same way I do all my hoppy ales; FWH and then torpedo.