Here's a pic of the top from that Myrtle OM we just finished.
Apologies for the out of focus pic but it's the only one I've got of this guitar.
The top of this is quite thin, 2.2mm, but it was a very stiff Engelmann top.
We left the braces a bit heftier than we normally do and because the top was so thin there is a small hardwood brace just behind the bridge plate to help with any bellying that might occur.
In fact we've chucked this brace on the two 12 strings we're building at the moment for the same reasons. I haven't seen that that extra mass in the centre of the soundboard has affected the tone detrimentally.
If you have a look at the intersection of the X and tone braces there is still about 2.5mm thickness on the tone brace. On a thicker top we would feather these down to nothing if we felt the top needed it.
And we simply tap the top until we get an even sustaining note consistently around the lower bout. I actually pay a lot of attention to the note that I get from tapping in the bridge plate area on the premise that this is where the strings are attached.
Both Dave and I cast an ear over the top before we commit to gluing it.
This is the brace that sits behind the bridge plate.
Bob