Unless someone has a hard answer, the best i've come up with is i will continue to ask the welders (maybe unreasonably so) to try for +/- 0.03" per weld and accept up to 0.06" per weld. My findings are below.
All valve specs either copy or directly reference ASME B16.10 with sites a +/- 0.06" (2mm) for valve face to face but all their valves look casted and machined (easier to hold tolerances). If welded this would be 2 welds giving +/- 0.03" per weld.
The welding spec PFI SE-3 shows at minimum 0.125" for a spool which would at minimum contain 2 welds giving +/- 0.06" per weld.