UKBASE,
The allowance for thin-out will depend on the material of construction for the spun heads. In general the more expensive the material, the more time and effort the head spinner can afford to put into the proccess and Q.A. of the heads.
Case in point: I have some on order at Brand X, where I needed (4) 0.310" Min. Hastelloy C-276 Heads at approx. 10-ft diameter, this required mill run material. The mill order for this thickness was in the range of 0.45".
Had these same heads been SA-516 Gr. 70, the nominal required might have been 0.625" (Stock not mill order). And the head manufacturer would spend considerably less time and care in spinning. The "extra" material costs less than the manufacturer's time it would take to control thin-out.
If there was 3/4" plate in stock in the required width, I might receive that instead of the head-manufacturer welding together the 5/8" thick.
As to Nominal Thickness required, it is best to specify your minimum (including corrosion and grinding allowances, if required) to the head fabricator/manufacturer and let them tell you what they can offer, and adjust your support/seismic design accordingly.
plates.