I've built up some wheels and the times I've had problems with spoke lengths is when I've trusted someone else (LBS,etc) to calculate them. Whenever I've used spocalc or the Dan Halem online calculator they've come out just right. Just make sure you measure the hubs right and know the rim dimenions before you use the program; the old carpenter's advice, "measure twice, cut once" applies. Most online spoke sellers won't accept returns for obvious reasons. Also, although the spocalc program has a database of rims and hubs, it never hurts to double check the measurements.