This is mostly for amusement's sake.
In the past and quite recently, I have gone through various books and the internet looking for this conversion information. Here are some of the various formulae I have come across in my travels:
for every cup of flour in the recipe, use either of
3 grams compressed fresh yeast
2 grams active dry yeast
1 gram instant active dry yeast
-Maggie Glezer, "Artisan Baking Across America"
Substitute twice as much (by weight) fresh yeast for the amount of dry yeast called for in the recipe.
-Daniel Leader, "Local Breads"
1 g fresh = 0.5 g active dry = 0.4 g instant
-Susan (Wild Yeast), wildyeastblog.com
2+1/2 tsp (one package) active dry yeast = 18 gm cake fresh yeast
-Carol Field, "The Italian Baker"
A .6-oz [17gm] cube of cake yeast is roughly equivalent to 1½ to 2 tsp. instant yeast or 2 to 2¼ tsp. active dry yeast.
-Sydny Carter, Yeast: The Basics, allrecipes.com
One .6 ounce [17 grams] cake is equivalent to 1 envelope [.25 ounce/7 grams] of dry yeast.
-Fleischmann's Yeast FAQ, breadworld.com
yeast, compressed . . . . 1 cake, 3/5 oz . . . . 1 package active dry yeast
-Irma S. Rombauer, Know your ingredients, Joy of Cooking
1 packed tablespoon of fresh or cake yeast=21 grams which=2-1/2 [8gm] teaspoons active dry (so for 100 grams fresh yeast use 1/4 cup + 1/2 teaspoon or 40 grams active dry)
-Rose Levy Beranbaum, realbakingwithrose.com
If you are substituting active dry yeast for instant yeast in a recipe, [...] add about 20 percent more yeast to the recipe than what is called for. [...] If you encounter a recipe that uses fresh yeast, divide the weight by 3 to calculate the proper amount of instant yeast to use.
-Yeast FAQ, thefreshloaf.com
Some years ago, with mixed up logic, I managed to work out the following formula. Remarkably, the bread I made rose beautifully.
2½ tsp (8gm) active dry yeast = 50gm fresh yeast
-me, my house
Depending on whose formula I use, to replace 50gm fresh yeast, I should use anywhere from 8 to 32.5 gm active dry yeast. (I think I have the arithmetic right with the various formulae: 32.5gm, 25gm, 22.5gm, 20gm, 17.5-20gm, 17gm, 8.3 OR 8gm active dry in place of 50 gm fresh yeast)
So. After all these contortions? I've decided that I'll use the higher amount of active dry to replace fresh yeast if there's lots of sugar in the recipe, but the lower amount if there's little sugar in the recipe.
edit: Ooops!!! I hit "save" by mistake. I MEANT to hit "preview". I think I've finished fixing things now. Have fun with the conversion chart!