There is alternate formula that you can use to calculate if you need to re-dimension the site or not. If assuming 50% of your traffic is blocked then add 5 Erls in your current traffic and recalculate utilization at whatever GOS you require at. If the new value is below 100% then you do not need to re-dimension. Check also on which layer you have blocking and more traffic, maybe in some cases you have blocking on 1800 layer while your maximum traffic is on 900 layer. Check for availability of 1800 TRXs etc.