If you are working to ASME Section III, you probably won't get much direction from the code.
I suggest you refer to AWS D1.X for some practical guidance on the fit up of structural elements. Keep in mind that AWS is not intended for pressure vessels, but there is still some useful information on joint details, fit up requirements, etc. that may be a reasonable basis of establishing fit up allowances where no direction is offered by ASME.
AWS D1.X X = 1 for Structural Welding Code/Steel
X = 2 for Structural Welding Code/Aluminum
X = 6 for Structural Welding Code/Stainless Steel
Best regards - Al