Butt welding is a very similar process to flash welding. One relies upon Joules Law for heat Butt Welding (Upset Welding).
Flash Welding relies upon the energy of an electrical arc to generate the heat.
The movement, upset and equipment are similar for the two processes.
Parameters for flash welding are published in:
AWS C1.1 – Recommended Practices for Resistance Welding
AWS C1.1 document states for upset welding the same welding parameters except those related to flash, can be used as a guide to develop upset welding schedules.
This is the source for start-up data. The data available in AWS C1.1 offers welding values that will make welds.
As in all resistance welding processes a starting point based on available data is chosen a little on the cold side. Then adjusted until the desired results are obtained.
References: AWS C1.1 - Recommended Practices for Resistance Welding
RWMA – Resistance Welding Manual 4th Edition