When going from ADY to IDY, I like to use a factor of 66. Just multiply the amount of ADY by .66 to get the correct IDY amount to use. Said another way, you will need to use only 66% as much IDY as you did ADY. For example, if you were using 6-ounces (by weight) of ADY, the amount of IDY needed to replace it would by 0.66 X 6 = 3,96-ounces (call it 4-ounces).
Also, remember, you don’t need to pre-hydrate the IDY, just add it directly to the flour.
And what you have left over in the bag, just roll the bag down on the yeast, and secure it with a rubber band, and store it on your ingredient shelf. No need to refrigerate, infact, refrigeration can result in condensation forming on the yeast, which and raise havoc with the stability of the yeast.
Tom Lehmann/The Dough Doctor