Sorry, you did it wrong. The roll of the die determines both the damage and the amount forward; Sam's ability reduces the damage, but does not increase the amount moved. Here's a quote from Chris Lawson's "Expanded Rules" page:
[Example: Sam, the current Ring-bearer decides to use the Ring. He rolls the Triple dot symbol on the die so first moves forward one space on the Corruption line (due to Sam's special skill) and then decides which Activity marker to move forward one space. If he rolled the Two cards symbol on the die he would have to discard a single card and then move an Activity marker forward two spaces.]
That page is found at:
for those who don't know already.