Be careful with this one. Two mistakes may have occurred in arriving at this result.
First off, just as the one-way distance between the mountain tops is half the round-trip distance measured by the laser so is the absolute uncertainty of the one-way distance half that for the round-trip distance.
Then make sure you convert the final answer correctly to percentages by multiplying the fractional error, i.e., the ratio "(absolute error)/(measurement value)", by 100.