How to Adjust Biscotti Recipe When Using Salted Pistachios
The only thing you can do is not add any additional salt, although I think you might still end up with a result that's too salty. As a commenter mentioned, you could get an indication of the amount of salt from the nutritional information on the packaging, but my suspicion is that in 3/4 of a cup of nuts there will be more than half a teaspoon of salt.
There is a question elsewhere on the site about unsalting salted nuts, where the consensus was that it wouldn't easily be possible. Tap as much salt off the nuts as possible before you use them.
If the nuts are going to get wet anyway (because there are other wet ingredients in your recipe that you haven't mentioned) then you might as well try to wash the water off; do it quickly, and pat the nuts try with paper towel immediately so they absorb as little water as possible. Tasting a few of them should tell you whether you've washed most of the salt off (in which case add it back according to your recipe) or not (in which case don't add any additional salt).