Projection welding schedules can be developed using standard spot welding schedules. For a single projection the weld time, current and force can be based upon available spot weld schedules. When welding multiple projections at one time and they are relatively far apart multiply the current times the number of projections. If the projections are close together and aligned, the total current will be reduced. Weld force is determined in the same manner. Weld time is the same no matter how many projections. Remember that in set up you always start on the cold side of power and work up to where you think you need to be to avoid damage to product and for the safety of people.
Weld schedules are available in the references below for some standard materials.
AWS and RWMA both publish spot welding schedules that can be used to develop a projection welding schedule. They also have published projection welding schedules for many materials.
Reference: AWS C1.1 Recommended Practices for Resistance Welding
RWMA Manual Section 3
Tuffaloy Product Inc. Catalog
CMW Inc. Resistance Welding Products Catalog