How wide is a plasm cut, I'm trying to work out wastage allowance on some 3 or 4mm steel. Thanks.?

That's a tough one bro, lemme think on it a bit

Kerf will depend on the type of plasma cutter, type and thickness of the steel, size of the cutting nozzle, and method (manual or CNC). There are variations within each process too. For example, when plasma cutting, the actual kerf width depends not only on the cutting current, but also on the torch height, speed, and gas settings.