The back is graduated to marked circles - the max thickness position is not centre of plate. It is generally the stop length marked up from the lower plate edge.
This puts max thickness/ stop position - about equal distance eitherside of the geometric centre. Thickness on this plate is 5.5mm max - 4mm - 3.3mm.
It is down to 2 - 2.2 mm in the upper and lower 'lungs' - this is a very hard and stiff piece of maple - and I was trying to aim for 175 gms weight.
I generally thin it until it 'feels' right - and the tap tone of the plate is around 280 hz.There is still material to come
off once the edges are finally sunk after purfling - so I look to lose maybe 5gms more on that.