No Kindding Jez & Erik,
The welding current or the secondary current is usually of the order of serveral thousand Amperes 10,000Amps is needed even in very low end B-Class spot welding machine. In some of machines my client makes it is 60,000Amps. And in very high end machines it is even larger. Measuring the current on secondary side will give exact value of welding current rather than measuring the primary side current and then assuming the secondary one.

Spot welding uses AC very low voltage and extremely high currents.

Regards
Prahlad Purohit.