That calculation might be correct if Meredith were standing On the cottage roof facing Nara's window at the time of her scream. The presumption though, using the prosecutions theory of the crime, is that Meredith was in her room at the time of this scream.
Meredith's window faces in the opposite direction and could not contribute to the sound reaching Nara's ear. The balcony door faces to the side and is shielded from Nara's view by the rest of the cottage. There is a small window in the large bath that faces Nara's window and sound from Meredith's bedroom can reach this window by bouncing down the hall, spreading out in the living room/kitchen and squeezing through the door to the bath. Then account for the glazing on this window. After that, it's a straight shot to Nara's window... almost. There is a small matter of the elevation of the road blocking the view. As I recall, Nara also claimed to have seen Amanda and Raffaele snuggling the day after. They would have been essentially in front of this window. But only the roof of the cottage was visible from Nara's window.
I'd love to see the Mythbusters bust this one. In their typical fashion, they wouldn't give up until they produced a loud enough noise for Nara to hear. Then the new tenant in the cottage could get double glazing when they replace all the windows that shattered