You also have to keep in mind that the crest (the edge over which the flow passes) isn't more than 1/8" thick or the nappe won't spring freely from the crest and will adhere to the downstream face of the weir plate.
If you are using the weir for flow measurement, the biggest problem is making sure the weir is set at the right elevation and that the weir pool (upstream body of water) is large enough to condition flow.