I have a beer that I going to brew, it is technically a gruit, b/c I am not going to use any hops. The original beer recipe (an American Hefe) calls for only one ounce of 4-6% alpha bittering hops and a 60 minute boil. The OG will be between 1.045 - 1.055. About how much wormwood should I add and how long should I boil?
There are no good bittering values ascribed to the many herbs used to design gruits. It comes down mostly to experimentation and the expectation that you won't make the best gruit on the first pass. I'd suggest looking for recipes for gruit with wormwood for guidance. Or start with a one gallon batch for test runs.
You could make two teas with equal volumes of water and equal mass of wormwood & hops in each one, and compare the bitterness levels. Try and get some other people to compare them as well, if you have willing tasters around. You may have to make a few batches to really get it figured out. If you have a few different types of hops, you could make several batches, so you could have a better basis for comparison. More bitter than Saaz doesn't really mean that much. More bitter than Columbus, and you know you really don't want to use much at all.