There's a bit of an art to cutting Powerspout nozzles, at least if you're trying to get a precise diameter of orifice so it delivers a precise flow.
I don't know how others do it but my technique is to mount a blank nozzle in the chuck of a wood lathe and serially slice off discs until the orifice diameter is exactly the size I want. Being hollow cones, the more you cut off, the bigger the orifice. I have the lathe turning as slowly as it will go and the knife needs to be as sharp as possible. It's rather too easy to take off too much and end up with an orifice which is bigger than you intended.
Measuring the resulting hole with real accuracy requires a "small hole gauge set". Such a set provides a series of gauges which open up a 'split ball' until its diameter is just capable of passing through the orifice you have cut. A micrometer is then used to measure the diameter of the split ball to the nearest 0.02mm.
The quality of the jet emerging from a nozzle is an important factor contributing to getting maximum efficiency from a runner, be it a pelton or a turgo runner. The surface of the jet should be smooth and the jet should remain compact and not break up too quickly after emerging from the orifice. To get such a jet, a good orifice is crucial and by cutting the nozzles in the way I do, a really clean edge is obtained which makes for the sort of jet you want.
The reason for cutting new nozzles, and bigger ones than I usually use, is because for the coming generating year 2015/16, I plan to use only the bottom jet. There is greater efficiency to be had if you can deliver the flow through one nozzle rather than two and if that nozzle is the bottom one. This is because the energy loss associated with nozzles is confined to just the one nozzle and being in the bottom position, splash and spray fall downward away from the runner, preventing rotational energy being sapped by the drag of the pelton rotating through water laden air.
EcoInnovation have in the past stipulated that power should not exceed 400W with single jet operation but Michael Lawley has indicated this is not a hard and fast rule. For my set up, I should be OK on one jet all the way up to design power of 750W (which is at 3 lps, and this should be provided by a nozzle orifice of 11.5 mm if my calculations are right !).
So that's why I've been cutting new and bigger nozzles: - to meet this plan for generation in the coming year. There's nothing like having a plan !