yeast conversion -- packet versus fresh?
Don't know whether this will help, but I just did some conversions myself for exactly that recipe. I was using instant yeast, not fresh, so I don't know if the measurements would be the same.
A packet of yeast is 1/4 of an ounce, about 2 teaspoons (it's definitely less than a tablespoon). I used a scale and measured out half an ounce of the instant and it worked perfectly in the brioche recipe. But the instant may weigh less than the fresh.