If your application requires a precise LD value, this heuristic isn’t for you, but the estimates are typically within about 0.05 of the true distance, which is more than enough accuracy for such tasks as:
Confirming suspected near-duplication.
Estimating how much two document vary.
Filtering through large numbers of documents to look for a near-match to some substantial block of text.
The estimation process is pretty interesting. Worth a read.