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I first found this ruling in David Easterling's player guide, which I believe was from right after Prophets. So, 1997?
Redemption has decimals? What about for cards like The Testimony? Can I choose e and 7-e?
I asked this a number of years ago, and I'm assuming the answer has not changed.
If a card's abilities are increase or decrease by half this can result in decimal or fractional strength and toughness. Do not round such results; use the fractional or decimal numbers as calculated.
Here's the part of the page on "Amplify." I think reproducing this small sample is OK under copyright laws, but if it isn't I'll remove it.
Since rarely will the half actually matter, if you are 5.5/8.5 you will still win against a 8/5 and lose to a 9/6 just like if you had been 5/8, so you would need another ability that interacts with the extra 1/2 to make it matter.
That's actually not entirely true. 5/8 Hero versus 8/5 EC is mutual destruction whereas 5.5/8.5 Hero versus 8/5 EC is not (the Hero is winning) and a 9/6 EC would be winning outright as well. It affects initiative as well as battle outcome.