In a rate-based flow control scheme, the important issue is how to determine congestion occurrence and congestion relief. The most common method is to set two thresholds of queue length, a high threshold and a low threshold. The values of these two thresholds seriously influence the system performance. Hence the authors present the concept of 'best area' to determine how to set the high and low thresholds to guarantee good performance (i.e. cell loss probability is zero and utilisation is one, if it is possible). Some rules are also given to prevent unnecessary cell loss and under-utilisation, when good performance is not achieved due to too many connections or too large a propagation delay.