Mill Tolerance for Ultrapipe

You will find the thickness tolerances within the ASME/ASTM specifications themselves, not on the U-1 form. Most plates are referenced back to SA-20 which has varying degrees of overtolerance dependant on the nominal thickness, but all the referenced plates have an undertolerance of 0.01".

However, please see paragraph UG-16 of ASME VIII-1:

"(c) Mill Undertolerance.

Plate material shall be ordered not thinner than the design thickness. Vessels made of plate furnished with an undertolerance of not more than the smaller value of 0.01 in. (0.25 mm) or 6% of the ordered thickness may be used at the full design pressure for the thickness ordered. If the specification to which the plate is ordered allows a greater undertolerance, the ordered thickness of the materials shall be sufficiently greater than the design thickness so that the thickness of the material furnished is not more than the smaller of 0.01 in. (0.25 mm) or 6% under the design thickness.

(d) Pipe Undertolerance.

If pipe or tube is ordered by its nominal wall thickness, the manufacturing undertolerance on wall thickness shall be taken into account except for nozzle wall reinforcement area requirements in accordance with UG-37 and UG-40. The manufacturing undertolerances are given in the several pipe and tube specifications listed in the applicable Tables in Subsection C. After the minimum wall thickness is determined, it shall be increased by an amount sufficient to provide the manufacturing undertolerance allowed in the pipe or tube specification."

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Lastly, you also have to factor in thinning due to forming and bending, and that is not an exact science.