1)
There is a shell:
Material: SA-516 Gr.70 Normalized
O.D. = 1219 mm
In the length = 1500 mm
minimum required thickness = 7 mm
corrosion allowance = 2 mm

According to ASME B&PVC 2007 Section II Part A I accept THICKNESSES TOLERANCE = 0,8 mm (SA-20/SA-20M; page 113; TABLE A2.1)

Design thickness = minimum required thickness + corrosion allowance + THICKNESSES TOLERANCE = 7+2+0,8 = 9,8 mm

Finally we specify plate thk 10 mm. Nominal thickness: 10 mm

This solution correct?

2)
There is a 2:1 ellipsoidal head:
Material: SA-516 Gr.70 Normalized
O.D. = 1219 mm
minimum required thickness = 7,1 mm
corrosion allowance = 2 mm

According to ASME B&PVC 2007 Section II Part A I accept THICKNESSES TOLERANCE = 0,8 mm (SA-20/SA-20M; page 113; TABLE A2.1)

Design thickness = minimum required thickness + corrosion allowance + THICKNESSES TOLERANCE = 7,1+2+0,8 = 9,9 mm
Finally we accept thickness on the standard ASME B16.9 – 2007 (Table 10; page 14)
Nominal thickness = 12,7 mm after forming

This solution correct?

Read carefully Table A2-1 it is +0.8 mm
Read Note 1: the minus tolerance is 0.3 mm

But better you read Sect.VIII Div.1 para UG-16(c), the undertolerance has not been rounded and the figure is -0.25 mm.
regards

What did you wish to tell?

I mean that the minus tolerance of plate thickness is not what you assumed to be and I am inviting you to read Sect.II. SA-20, Table A2-1: Note 1: the minus tolerance is 0.3 mm

and

Sect.VIII Div.1 para UG-16(c), the undertolerance has not been rounded and the figure is -0.25 mm.
So the minus tolerance is -0.25 mm or 6% (the smaller)

Mauro