When welding a flange, fitting or a piece of pipe together the gap spacing for welding them is established by the Welding Qualification. API Std 1104 (Standard for Welding of Pipelines and Related Facilities) calls for a 1/16" space. On a socket weld, its 1/16" and can be established with a special spring. The offset between welds cannot be greater than 1/8".
When welding a slip-on flange, the pipe or fitting is set back from the face of the flange a distance equal to the wall thickness -0" +1/16".