One of the tricks Larry uses during top tuning is to graduate the top thickness. The top is thinned at the edges and then faired (or is it 'fared'?) in to the rest of the top. Thus the top has a wedge-like profile from center to edge in the lower bout, thickest in the middle, gradually getting thinner toward the edges. Typical reduction at the edges is .005"-.010". The amount of thinning is determined by how the top is responding to the brace carving and a good dose of luthier's intuition.
I can tell you this top responded somewhat to brace reductions, but was sounding tight, being a bit "stubborn". Larry had taken the brace heights well below Martin spec (used simply as a reference) and done quite a bit of profiling. Neither of us were surprised as this is a very stiff set of spruce. Nonetheless, it was time for some edge thinning. The resonance bloomed after the edge thinning. Pretty amazing.
The top is marked to indicate areas to be thinned based on thickness measurements.
An index line of the desired depth is routed into the top.
The top is then sanded "into" the index groove (sanding block shown above). Typically a couple of grams of material is removed.
Look ma...no index groove.
