You soooooooooooooo suck at math.
160/2.5 is not 12, it's 64. At least in this universe. On the other hand, 30/2.5 is 12.
Don't feel too bad, other people soooooooooooo suck at gelatin. Mass for mass not all gelatines are created equal. Some are more efficient than others -- that is, they can gel a greater amount of liquid.
There is a formula for converting various forms of sheet to various forms of powder. Here goes:
Weight (Bloom 2) = Weight (Bloom 1) x square root (Bloom1/Bloom2)
You're undoubtedly much better at pastry than I am, but I'm pretty good at 'rithmetic. So I worked it out for you. Mass for mass, Knox gelatin powder is roughly 1.2X more efficient than 160 bloom sheet gelatin. Conversely (and reciprocally), it might also help you to know that 160 bloom sheet gelatin is roughly 0.8X as efficient as Knox gelatin powder.
Thus, to answer your specific question directly, 30 grams of 160 sheet is roughly equivalent to 25 grams of Knox powder.
It doesn't do to rely on conversion numbers or gelatin amounts too carefully. There's a certain amount of by God and by Gosh in most recipes, and they usually specify a little more than is absolutely needed just to be sure. Unless you're making something with a very soft texture which will be very carefully held (pannacotta, fine dining, e.g.) it's better to continue with "better safe than sorry."
When cooking with gelatin conversions a little too much is better than even a bit too little.
25g is on the money. If you measure 30 grams instead of 25, that won't be too bad. Just under 20g -- which is your proposal -- might be a little risky.
Hope this helps,
PS. Since you asked, Pete's "225" is the bloom number for Knox powdered gelatin.
Edited by boar_d_laze - 8/9/10 at 8:00pm