*Desired Thickness is in ZBrush units ?
These are the units that appear top left when using the 'Transpose' tool and in '3DPrintExporter'.
To get precise measurements, relate these units to real-world units. 1 = 1mm for example.
*Find your Output Scale in Tool>Export>Output Scale ?
Increasing your 'Output Scale' allows you to work at larger object scales without affecting how Dynamesh density is calculated.
Thus allowing you to create larger shell thicknesses.
*Keep in mind the Dynamesh Thickness max value is 128 ?
Working at a higher 'Output Scale' can allow you to more easily create larger thicknesses without having to reduce your 'Dynamesh Resolution'.
Increase 'Output Scale' by increments of 10 to make translating scale between different softwares simpler.
*Resulting Dynamesh Thickness value is rounded upwards, as ZBrush only accepts integers ?
If rounded to the nearest value, you could end up with a value below your desired thickness.